Illumination device and method for controlling an illumination device over changes in drive current and temperature

ABSTRACT

An illumination device and method is provided herein for controlling individual light emitting diodes (LEDs) in an LED illumination device, so that a desired luminous flux and a desired chromaticity of the device can be maintained over changes in drive current and temperature. According to one embodiment, the illumination device comprises a plurality of emission LEDs, a storage medium, an LED driver and receiver circuit and a control circuit. The storage medium may store a table of calibration values correlating forward voltage and drive current to chromaticity and luminous flux at a plurality of temperatures for each of the plurality of emission LEDs. The LED driver and receiver circuit may apply respective drive currents to the emission LEDs to produce substantially continuous illumination, and may periodically turn the emission LEDs off to measure operating forward voltages that develop across each emission LED. The control circuit may determine whether a target luminance setting or a target chromaticity setting for the illumination device has changed, and if so, may determine new respective drive currents needed to achieve the target luminance setting and the target chromaticity setting using the operating forward voltages measured across each emission LED, the table of calibration values and one or more interpolation techniques.

RELATED APPLICATIONS

This application is related to commonly assigned U.S. patent applicationSer. Nos. 13/970,944, now issued as U.S. Pat. No. 9,237,620;13/970,964and 13/970,990; U.S. Provisional Application No. 61/886,471;and U.S. patent application Ser. No. 14/097,339. The entirety of theseapplications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to illumination devices comprising a plurality oflight emitting diodes (LEDs) and, more particularly, to illuminationdevices and methods for controlling an illumination device, so as toobtain a desired luminous flux and a desired chromaticity of the deviceover changes in drive current and temperature.

2. Description of the Relevant Art

The following descriptions and examples are provided as background onlyand are intended to reveal information that is believed to be ofpossible relevance to the present invention. No admission is necessarilyintended, or should be construed, that any of the following informationconstitutes prior art impacting the patentable character of the subjectmatter claimed herein.

Lamps and displays using LEDs (light emitting diodes) for illuminationare becoming increasingly popular in many different markets. LEDsprovide a number of advantages over traditional light sources, such asincandescent and fluorescent light bulbs, including low powerconsumption, long lifetime, no hazardous materials, and additionalspecific advantages for different applications. When used for generalillumination, LEDs provide the opportunity to adjust the color (e.g.,from white, to blue, to green, etc.) or the color temperature (e.g.,from “warm white” to “cool white”) to produce different lightingeffects.

Although LEDs have many advantages over conventional light sources, onedisadvantage of LEDs is that their output characteristics (e.g.,luminous flux and chromaticity) vary over changes in drive current,temperature and over time as the LEDs age. These effects areparticularly evident in multi-colored LED illumination devices, whichcombine a number of differently colored emission LEDs into a singlepackage.

An example of a multi-colored LED illumination device is one in whichtwo or more different colors of LEDs are combined within the samepackage to produce white or near-white light. There are many differenttypes of white light lamps on the market, some of which combine red,green and blue (RGB) LEDs, red, green, blue and yellow (RGBY) LEDs,phosphor-converted white and red (WR) LEDs, RGBW LEDs, etc. By combiningdifferent colors of LEDs within the same package, and driving thedifferently colored LEDs with different drive currents, these lamps maybe configured to generate white or near-white light within a wide gamutof color points or correlated color temperatures (CCTs) ranging from“warm white” (e.g., roughly 2600K-3700K), to “neutral white” (e.g.,3700K-5000K) to “cool white” (e.g., 5000K-8300K). Some multi-colored LEDillumination devices also enable the brightness and/or color of theillumination to be changed to a particular set point. These tunableillumination devices should all produce the same color and colorrendering index (CRI) when set to a particular dimming level andchromaticity setting (or color set point) on a standardized chromacitydiagram.

A chromaticity diagram maps the gamut of colors the human eye canperceive in terms of chromacity coordinates and spectral wavelengths.The spectral wavelengths of all saturated colors are distributed aroundthe edge of an outlined space (called the “gamut” of human vision),which encompasses all of the hues perceived by the human eye. The curvededge of the gamut is called the spectral locus and corresponds tomonochromatic light, with each point representing a pure hue of a singlewavelength. The straight edge on the lower part of the gamut is calledthe line of purples. These colors, although they are on the border ofthe gamut, have no counterpart in monochromatic light. Less saturatedcolors appear in the interior of the figure, with white and near-whitecolors near the center.

In the 1931 CIE Chromaticity Diagram, colors within the gamut of humanvision are mapped in terms of chromaticity coordinates (x, y). Forexample, a red (R) LED with a peak wavelength of 625 nm may have achromaticity coordinate of (0.69, 0.31), a green (G) LED with a peakwavelength of 528 nm may have a chromaticity coordinate of (0.18, 0.73),and a blue (B) LED with a peak wavelength of 460 nm may have achromaticity coordinate of (0.14, 0.04). The chromaticity coordinates(i.e., color points) that lie along the blackbody locus obey Planck'sequation, E(λ)=Aλ⁻⁵/(e^((B/T))−1). Color points that lie on or near theblackbody locus provide a range of white or near-white light with colortemperatures ranging between approximately 2500K and 10,000K. Thesecolor points are typically achieved by mixing light from two or moredifferently colored LEDs. For example, light emitted from the RGB LEDsshown in FIG. 1 may be mixed to produce a substantially white light witha color temperature in the range of about 2500K to about 5000K.

Although an illumination device is typically configured to produce arange of white or near-white color temperatures arranged along theblackbody curve (e.g., about 2500K to 5000K), some illumination devicesmay be configured to produce any color within the color gamut 18(triangle) formed by the individual LEDs (e.g., RGB). The chromaticitycoordinates of the combined light, e.g., (0.437, 0.404) for 3000K whitelight, define the target chromaticity or color set point at which thedevice is intended to operate. In some devices, the target chromaticityor color set point may be changed by altering the ratio of drivecurrents supplied to the individual LEDs.

In general, the target chromaticity of the illumination device may bechanged by adjusting the drive current levels (in current dimming) orduty cycle (in PWM dimming) supplied to one or more of the emissionLEDs. For example, an illumination device comprising RGB LEDs may beconfigured to produce “warmer” white light by increasing the drivecurrent supplied to the red LEDs and decreasing the drive currentssupplied to the blue and/or green LEDs. Since adjusting the drivecurrents also affects the lumen output and temperature of theillumination device, the target chromaticity must be carefullycalibrated and controlled to ensure that the actual chromaticity equalsthe target value. Most prior art illumination devices fail to provide anaccurate calibration and compensation method for controlling the colorof the illumination device.

Some prior art illumination devices also provide dimming capabilities,i.e., the ability to change the brightness or luminous flux output fromthe emission LEDs, in addition to (or instead of) color tuning. In mostcases, the dimming level is changed by adjusting the drive currentlevels (in current dimming) or the duty cycle of the drive currents (inPWM dimming) supplied to all emission LEDs to produce a target dimminglevel. However, adjusting the supplied drive currents changes thechromaticity of the illumination, and this change in chromaticitydiffers for different LED devices and different dimming methods. Forexample, the chromaticity of an RGB LED illumination device may changerather significantly with changes drive current level and duty cycle,while the chromaticity of a phosphor-converted white LED illuminationdevice is more consistent. In order to maintain a consistent targetchromaticity, a range of target chromaticity values must be carefullycalibrated over a range of target dimming levels.

In practice, the lumen output and chromaticity produced by prior artillumination devices often differs from the target dimming level andtarget chromaticity setting, due to changes in temperature and over timeas the LEDs age. In general, changes in temperature affect the lumenoutput and chromaticity of all phosphor converted and non-phosphorconverted LEDs. While prior art devices may perform some level oftemperature compensation, they fail to provide accurate results byfailing to recognize that temperature affects the lumen output andchromaticity of different colors of LEDs differently. Moreover, theseprior art devices fail to account for chromaticity shifts in theillumination produced by phosphor converted LEDs, which result fromphosphor aging. As a consequence, these prior art devices cannotmaintain a desired luminous flux and a desired chromaticity for an LEDillumination device over operating conditions and over the lifetime ofthe illumination device.

A need remains for improved illumination devices and methods forcalibrating and compensating individual LEDs within an LED illuminationdevice, so as to accurately maintain a desired luminous flux and adesired chromaticity for the illumination device over changes intemperature, changes in drive current and over time as the LEDs age.This need is particularly warranted in multi-color LED illuminationdevices, since different colors of LEDs are affected differently bytemperature and age, and in tunable illumination devices that enable thetarget dimming level and/or the target chromaticity setting to bechanged by adjusting the drive currents supplied to one or more of theLEDs, since changes in drive current inherently affect the lumen output,color and temperature of the illumination device.

SUMMARY OF THE INVENTION

The following description of various embodiments of an illuminationdevice and a method for controlling an illumination device is not to beconstrued in any way as limiting the subject matter of the appendedclaims.

According to one embodiment, a method is provided herein for controllingan LED illumination device, so that a desired luminous flux and adesired chromaticity of the device can be maintained over changes intemperature and drive current. In general, the illumination devicedescribed herein may include a plurality of emission LEDs, or aplurality of chains of emission LEDs. For the sake of simplicity, theterm “LED” will be used herein to refer to a single LED or a chain ofserially connected LEDs supplied with the same drive current.

According to one embodiment, the method described herein may begin byapplying respective drive currents to the plurality of emission LEDs todrive the plurality of emission LEDs substantially continuously toproduce illumination, periodically turning the plurality of emissionLEDs off for short durations of time, and measuring a forward voltagepresently developed across each emission LED by applying a non-operativedrive current to each emission LED, one LED at a time, during the shortdurations of time the plurality of emission LEDs are periodically turnedoff. As used herein, a non-operative drive current may be described as adrive current level, which is insufficient to produce significantillumination from the LED, and may range between approximately 0.1 mAand approximately 10 mA.

The method described herein may further include determining chromaticityvalues that are expected for each emission LED using the forward voltagemeasured across each emission LED, the respective drive current suppliedto each emission LED, a table of stored calibration values correlatingforward voltage and drive current to chromaticity at a plurality ofdifferent temperatures, and one or more interpolation techniques. Insome embodiments, the method may further include monitoring a targetluminance setting and a target chromaticity setting of the illuminationdevice, and performing the steps of measuring and determining only whena change is detected in at least one of the target luminance and targetchromaticity settings.

In general, the table of stored calibration values may includechromaticity calibration values corresponding to a CIE 1931 XYZ colorspace, a CIE 1931 RGB color space, a CIE 1976 LUV color space, orvarious other RGB color spaces (e.g., sRGB, Adobe RGB, etc.). Althoughthe compensation method described herein may utilize only x and ychromaticity calibration values, one skilled in the art would understandhow chromaticity values from other color spaces could be alternativelyacquired and used in the methods described herein. As such, thecompensation method described herein and recited in the claims isconsidered to encompass chromaticity calibration values from any colorspace that can be used to describe the gamut of an LED illuminationdevice comprising substantially any combination of emission LEDs asdescribed herein.

According to one embodiment, the table of stored calibration values mayinclude, for each emission LED, a first plurality of x chromaticityvalues and a first plurality of y chromaticity values, which werepreviously detected from the emission LED upon applying a plurality ofdifferent drive currents to the emission LED during a calibration phasewhen the emission LED was subjected to a first temperature, and a secondplurality of x chromaticity values and a second plurality of ychromaticity values, which were previously detected from the emissionLED upon applying the plurality of different drive currents to theemission LED during the calibration phase when the emission LED wassubjected to a second temperature.

In addition, the table of calibration values may include a firstplurality of forward voltage values and a second plurality of forwardvoltage values for each emission LED. The first plurality of forwardvoltage values may be ones, which were previously measured across theemission LED upon applying a non-operative drive current to the emissionLED, before, during or after each of the different drive currents wasapplied to the emission LED, when the emission LED was subjected to thefirst temperature. The second plurality of forward voltage values may beones, which were previously measured across the emission LED uponapplying a non-operative drive current to the emission LED, before,during or after each of the different drive currents was applied to theemission LED, when the emission LED was subjected to the secondtemperature.

In one embodiment, an expected x chromaticity value for each emissionLED may be determined by calculating a third plurality of x chromaticityvalues corresponding to the forward voltage measured across the emissionLED by interpolating between the first plurality of x chromaticityvalues and the second plurality of x chromaticity values, generating arelationship between the third plurality of x chromaticity values, ifthe respective drive current supplied to the emission LED differs fromone of the plurality of different drive currents applied to the emissionLED during the calibration phase, and determining, from the generatedrelationship, an expected x chromaticity value for the emission LEDcorresponding to the respective drive current applied to the emissionLED.

In one embodiment, an expected y chromaticity value for each emissionLED may be determined by calculating a third plurality of y chromaticityvalues corresponding to the forward voltage measured across the emissionLED by interpolating between the first plurality of y chromaticityvalues and the second plurality of y chromaticity values, generating arelationship between the third plurality of y chromaticity values, ifthe respective drive current supplied to the emission LED differs fromone of the plurality of different drive currents applied to the emissionLED during the calibration phase, and determining, from the generatedrelationship, an expected y chromaticity value for the emission LEDcorresponding to the respective drive current applied to the emissionLED.

In some cases, the steps of calculating a third plurality of xchromaticity values and a third plurality of y chromaticity values mayeach comprise using a linear interpolation technique to interpolatebetween the first and second plurality of x and y chromaticity values.In some cases, the step of generating a relationship may compriseapplying a higher-order interpolation to the third plurality of x and ychromaticity values to generate a non-linear relationship between x andy chromaticity and drive current at the forward voltage measured acrossthe emission LED. In other cases, the step of generating a relationshipmay comprise applying a piece-wise linear interpolation to the thirdplurality of x and y chromaticity values to approximate a non-linearrelationship between x and y chromaticity and drive current at theforward voltage measured across the emission LED. In yet other cases,the step of generating a relationship may comprise assuming a typicalcurvature from data sheets provided by an LED manufacturer.

In some embodiments, the method may further include calculating arelative luminous flux needed from each emission LED to achieve a targetluminance setting and a target chromaticity setting of the illuminationdevice. In general, the relative luminous flux for each emission LED maybe calculated using the target luminance setting, the targetchromaticity setting, the expected x chromaticity value and the expectedy chromaticity value determined from the generated relationships.

In some embodiments, the method may further include determining a drivecurrent needed to achieve the relative luminous flux calculated for eachemission LED using the forward voltage measured across each emissionLED, the table of stored calibration values, and one or moreinterpolation techniques, and driving each emission LED with thedetermined drive current to produce illumination having the calculatedrelative luminous flux.

In addition to correlating forward voltage and drive current tochromaticity at a plurality of different temperatures, the table ofstored calibration values may further correlate forward voltage anddrive current to luminous flux at the plurality of differenttemperatures. For example, the table of stored calibration values mayinclude, for each emission LED, a first plurality of luminous fluxvalues, which were previously detected from the emission LED uponapplying the plurality of different drive currents to the emission LEDduring the calibration phase when the emission LED was subjected to thefirst temperature, and a second plurality of luminous flux values, whichwere previously detected from the emission LED upon applying theplurality of different drive currents to the emission LED during thecalibration phase when the emission LED was subjected to the secondtemperature.

For each emission LED, the step of determining a drive current mayinclude calculating a third plurality of luminous flux valuescorresponding to the forward voltage measured across the emission LED byinterpolating between the first plurality of luminous flux values andthe second plurality of luminous flux values, generating a relationshipbetween the third plurality of luminous flux values, if the relativeluminous flux calculated for the emission LED differs from one of thethird plurality of luminous flux values, and determining the drivecurrent needed to achieve the calculated relative luminous flux byselecting, from the generated relationship, a drive currentcorresponding to the calculated relative luminous flux.

In some cases, the third plurality of luminous flux values may becalculated by using a linear interpolation technique or a non-linearinterpolation technique to interpolate between the first and secondplurality of luminous flux values. In general, selection between thelinear interpolation technique and the non-linear interpolationtechnique is independently made for each emission LED based on a colorof the emission LED.

In some cases, the relationship may be generated by applying ahigher-order interpolation to the third plurality of luminous fluxvalues to generate a non-linear relationship between luminous flux anddrive current. In other cases, the relationship may be generated byapplying a piece-wise linear interpolation to the third plurality ofluminous flux values to approximate a non-linear relationship betweenluminous flux and drive current. In yet other cases, the relationshipmay be generated by assuming a typical curvature from data sheetsprovided by an LED manufacturer.

According to another embodiment, an illumination device is providedherein with a plurality of emission light emitting diodes (LEDs), astorage medium, an LED driver and receiver circuit and a controlcircuit. The plurality of light emitting diode (LED) chains may begenerally configured to produce illumination for the illuminationdevice. In some embodiments, each emission LED may be configured forproducing illumination at a different peak wavelength. In otherembodiments, however, one or more of the emission LEDs may be configuredfor producing illumination at the same peak emission wavelength.

The storage medium may be generally configured for storing a table ofcalibration values correlating forward voltage and drive current tochromaticity and luminous flux at a plurality of temperatures for eachof the plurality of emission LEDs. In general, the table of calibrationvalues stored within the storage medium may include chromaticitycalibration values corresponding to a CIE 1931 XYZ color space, a CIE1931 RGB color space, a CIE 1976 LUV color space, or various other RGBcolor spaces (e.g., sRGB, Adobe RGB, etc.).

According to one embodiment, the table of calibration values mayinclude, for each emission LED, a first plurality of x chromaticityvalues and a first plurality of y chromaticity values, which werepreviously detected from the emission LED upon applying a plurality ofdifferent drive currents to the emission LED during a calibration phasewhen the emission LED was subjected to a first temperature. The table ofcalibration values may also include, for each emission LED, a secondplurality of x chromaticity values and a second plurality of ychromaticity values, which were previously detected from the emissionLED upon applying the plurality of different drive currents to theemission LED during the calibration phase when the emission LED wassubjected to a second temperature.

In addition, the table of calibration values may include a firstplurality of forward voltage values and a second plurality of forwardvoltage values for each emission LED. The first plurality of forwardvoltage values may be ones, which were previously measured across theemission LED upon applying a non-operative drive current to the emissionLED, before, during or after each of the different drive currents wasapplied to the emission LED, when the emission LED was subjected to thefirst temperature. The second plurality of forward voltage values may beones, which were previously measured across the emission LED uponapplying a non-operative drive current to the emission LED, before,during or after each of the different drive currents was applied to theemission LED, when the emission LED was subjected to the secondtemperature.

In addition, the table of calibration values may further include a firstplurality of luminous flux values and a second plurality of luminousflux values for each emission LED. The first plurality of luminous fluxvalues may be ones, which were previously detected from the emission LEDupon applying the plurality of different drive currents to the emissionLED during the calibration phase when the emission LED was subjected tothe first temperature. The second plurality of luminous flux values maybe ones, which were previously detected from the emission LED uponapplying the plurality of different drive currents to the emission LEDduring the calibration phase when the emission LED was subjected to thesecond temperature.

The LED driver and receiver circuit may be generally configured forapplying respective drive currents to the plurality of emission LEDs todrive the plurality of emission LEDs substantially continuously toproduce illumination, periodically turning the plurality of emissionLEDs off for short durations of time, and applying a non-operative drivecurrent to each emission LED, one LED at a time, during the shortdurations of time to measure an operating forward voltage developedacross each emission LED. As used herein, a non-operative drive currentmay be described as a drive current level, which is insufficient toproduce significant illumination from the LED, and may range betweenapproximately 0.1 mA and approximately 10 mA.

In some embodiments, the illumination device may further include a phaselocked loop (PLL), which is coupled to an AC mains and configured forproducing a timing signal in synchronization with a frequency of the ACmains. In such embodiments, the timing signal may be supplied to the LEDdriver and receiver circuit for periodically turning the plurality ofemission LEDs off for the short durations of time.

The control circuit may be coupled to the LED driver and receivercircuit and the storage medium. In some embodiments, the control circuitmay be configured for determining whether a target luminance setting ora target chromaticity setting for the illumination device has changed,and if so, for determining new respective drive currents needed toachieve the target luminance setting and the target chromaticity settingusing the operating forward voltages measured across each emission LED,the table of calibration values and one or more interpolationtechniques.

In some embodiments, the illumination device may further include atemperature sensor, which is configured for detecting an ambienttemperature surrounding the plurality of emission LEDs. If the targetluminance setting and the target chromaticity setting for theillumination device has not changed, the control circuit may beconfigured for determining the respective drive currents needed toachieve the target luminance setting and the target chromaticity settingonly when the ambient temperature changes by a specified amount.

For each LED chain, the control circuit may be configured forcalculating a third plurality of x chromaticity values corresponding tothe operating forward voltage measured across the emission LED byinterpolating between the first plurality of x chromaticity values andthe second plurality of x chromaticity values, generating a relationshipbetween the third plurality of x chromaticity values, if the respectivedrive current applied to the emission LED differs from one of theplurality of different drive currents applied to the emission LED duringthe calibration phase, and determining, from the generated relationship,an expected x chromaticity value for the emission LED corresponding tothe respective drive current applied to the emission LED. For eachemission LED, the control circuit may be further configured forcalculating a third plurality of y chromaticity values corresponding tothe operating forward voltage measured across the emission LED byinterpolating between the first plurality of y chromaticity values andthe second plurality of y chromaticity values, generating a relationshipbetween the third plurality of y chromaticity values, if the respectivedrive current applied to the emission LED differs from one of theplurality of different drive currents applied to the emission LED duringthe calibration phase, and determining, from the generated relationship,an expected y chromaticity value for the emission LED corresponding tothe respective drive current applied to the emission LED.

In some cases, the control circuit may be configured for calculating thethird plurality of x and y chromaticity values for each emission LED byusing a linear interpolation technique to interpolate between the firstand second plurality of x and y chromaticity values corresponding to theemission LED. In some cases, the control circuit may be configured forapplying a higher-order interpolation to the third plurality of x and ychromaticity values to generate non-linear relationships between x and ychromaticity and drive current for each emission LED. In other cases,the control circuit may be configured for applying a piece-wise linearinterpolation to the third plurality of x and y chromaticity values toapproximate non-linear relationships between x and y chromaticity anddrive current for each emission LED.

In some embodiments, the control circuit may be further configured forcalculating a relative luminous flux needed from each emission LED toachieve the target luminance setting and the target chromaticitysetting. In general, the control circuit may calculate the relativeluminous flux for each emission LED using the target luminance settingand the target chromaticity setting, as well as the expected x and ychromaticity values determined from the generated relationships.

In some embodiments, the control circuit may be further configured fordetermining a drive current needed to achieve the relative luminous fluxcalculated for each emission LED using the operating forward voltagemeasured across the emission LED, the table of stored calibrationvalues, and one or more interpolation techniques. In such embodiments,the LED driver and receiver circuit may be further configured fordriving each emission LED with the determined drive current to produceillumination having the calculated relative luminous flux.

In some embodiments, the control circuit may be configured fordetermining the drive current needed to achieve the relative luminousflux calculated for each emission LED by calculating a third pluralityof luminous flux values corresponding to the operating forward voltagemeasured across the emission LED by interpolating between the firstplurality of luminous flux values and the second plurality of luminousflux values, generating a relationship between the third plurality ofluminous flux values, if the relative luminous flux calculated for theemission LED differs from one of the third plurality of luminous fluxvalues, and determining the drive current needed to achieve thecalculated relative luminous flux by selecting, from the generatedrelationship, a drive current corresponding to the calculated relativeluminous flux.

In some cases, the control circuit may be configured for calculating thethird plurality of luminous flux values for each emission LED by using alinear interpolation technique or a non-linear interpolation techniqueto interpolate between the first and second plurality of luminous fluxvalues corresponding to the emission LED. In general, selection betweenthe linear interpolation technique and the non-linear interpolationtechnique is independently made for each emission LED based on a colorof the emission LED.

In some cases, the control circuit may be configured to generate therelationship by applying a higher-order interpolation to the thirdplurality of luminous flux values to generate a non-linear relationshipbetween luminous flux and drive current for each emission LED. In othercases, the control circuit may be configured to generate therelationship by applying a piece-wise linear interpolation to the thirdplurality of luminous flux values to approximate a non-linearrelationship between luminous flux and drive current for each emissionLED. In yet other cases, the control circuit may be configured forgenerating the relationship by assuming a typical curvature from datasheets provided by a manufacturer of the emission LED.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings.

FIG. 1 is a graph of the 1931 CIE chromaticity diagram illustrating thegamut of human color perception and the gamut achievable by anillumination device comprising a plurality of multiple color LEDs (e.g.,red, green and blue);

FIG. 2 is a graph illustrating the non-linear relationship betweenrelative luminous flux and junction temperature for white, blue andgreen LEDs;

FIG. 3 is a graph illustrating the substantially more non-linearrelationship between relative luminous flux and junction temperature forred, red-orange and yellow (amber) LEDs;

FIG. 4 is a graph illustrating the non-linear relationship betweenrelative luminous flux and drive current for red and red-orange LEDs;

FIG. 5 is a graph illustrating the substantially more non-linearrelationship between relative luminous flux and drive current for white,blue and green LEDs;

FIG. 6 is a flow chart diagram of an improved method for calibrating anillumination device comprising a plurality of LEDs and one or morephotodetectors, in accordance with one embodiment of the invention;

FIG. 7 is a chart illustrating an exemplary table of calibration valuesthat may be obtained in accordance with the calibration method of FIG. 6and stored within the illumination device;

FIG. 8 is a flowchart diagram of an improved compensation method, inaccordance with one embodiment of the invention;

FIG. 9 is an exemplary timing diagram for an illumination devicecomprising four emission LEDs, illustrating the periodic intervalsduring which measurements (e.g., emitter forward voltage) are obtainedfrom each emission LED, one LED at a time;

FIG. 10 is a graphical representation depicting how one or moreinterpolation technique(s) may be used in the compensation method ofFIG. 8 to determine the drive current needed to produce a desiredluminous flux for a given LED using the calibration values obtainedduring the calibration method of FIG. 6 and stored within theillumination device;

FIG. 11 is a graphical representation depicting how one or moreinterpolation technique(s) may be used in the compensation method ofFIG. 8 to determine the expected x chromaticity value for a given LEDusing the present forward voltage, the present drive current and thecalibration values obtained during the calibration method of FIG. 6 andstored within the illumination device;

FIG. 12 is a graphical representation depicting how one or moreinterpolation technique(s) may be used in the compensation method ofFIG. 8 to determine the expected y chromaticity value for a given LEDusing the present forward voltage, the present drive current and thecalibration values obtained during the calibration method of FIG. 6 andstored within the illumination device;

FIG. 13 is a flowchart diagram of an improved compensation method, inaccordance with another embodiment of the invention;

FIG. 14 is an exemplary timing diagram for an illumination devicecomprising four emission LEDs, illustrating the periodic intervalsduring which measurements are obtained from the one or morephotodetectors (e.g., induced photocurrent and detector forward voltage)and from each emission LED, one LED at a time (e.g., emitter forwardvoltage);

FIG. 15 is a graphical representation depicting how one or moreinterpolation technique(s) may be used in the compensation method ofFIG. 13 to determine the expected photocurrent value for a given LEDusing the present forward voltage, the present drive current and thecalibration values obtained during the calibration method of FIG. 6 andstored within the illumination device;

FIG. 16A is a photograph of an exemplary illumination device;

FIG. 16B is a computer generated image showing a top view of anexemplary emitter module that may be included within the exemplaryillumination device of FIG. 16A;

FIG. 17A is a photograph of another exemplary illumination device;

FIG. 17B is a computer generated image showing a top view of anexemplary emitter module that may be included within the exemplaryillumination device of FIG. 17A;

FIG. 18A is a side view of an improved emitter module, according to oneembodiment of the invention;

FIG. 18B is a side view of an improved emitter module, according toanother embodiment of the invention;

FIG. 19 is an exemplary block diagram of circuit components that may beincluded within an illumination device, according to one embodiment ofthe invention;

FIG. 20 is an exemplary block diagram of an LED driver and receivercircuit that may be included within the illumination device of FIG. 19,according to one embodiment of the invention; and

FIG. 21 is an exemplary graph depicting how the spectrum of a phosphorconverted LED may be divided into two portions, and showing how thephosphor efficiency decreases as the phosphor ages.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An LED generally comprises a chip of semiconducting material doped withimpurities to create a p-n junction. As in other diodes, current flowseasily from the p-side, or anode, to the n-side, or cathode, but not inthe reverse direction. Charge-carriers—electrons and holes—flow into thejunction from electrodes with different voltages. When an electron meetsa hole, it falls into a lower energy level, and releases energy in theform of a photon (i.e., light). The wavelength of the light emitted bythe LED, and thus its color, depends on the band gap energy of thematerials forming the p-n junction of the LED.

Red and yellow LEDs are commonly composed of materials (e.g., AlInGaP)having a relatively low band gap energy, and thus produce longerwavelengths of light. For example, most red and yellow LEDs have a peakwavelength in the range of approximately 610-650 nm and approximately580-600 nm, respectively. On the other hand, green and blue LEDs arecommonly composed of materials (e.g., GaN or InGaN) having a larger bandgap energy, and thus, produce shorter wavelengths of light. For example,most green and blue LEDs have a peak wavelength in the range ofapproximately 515-550 nm and approximately 450-490 nm, respectively.

In some cases, a “white” LED may be formed by covering or coating, e.g.,a blue LED having a peak emission wavelength of about 450-490 nm with aphosphor (e.g., YAG), which down-converts the photons emitted by theblue LED to a lower energy level, or a longer peak emission wavelength,such as about 525 nm to about 600 nm. In some cases, such an LED may beconfigured to produce substantially white light having a correlatedcolor temperature (CCT) of about 3000K. However, a skilled artisan wouldunderstand how different colors of LEDs and/or different phosphors maybe used to produce a “white” LED with a potentially different CCT.

When two or more differently colored LEDs are combined within a singlepackage, the spectral content of the individual LEDs are combined toproduce blended light. In some cases, differently colored LEDs may becombined to produce white or near-white light within a wide gamut ofcolor points or CCTs ranging from “warm white” (e.g., roughly2600K-3000K), to “neutral white” (e.g., 3000K-4000K) to “cool white”(e.g., 4000K-8300K). Examples of white light illumination devicesinclude, but are not limited to, those that combine red, green and blue(RGB) LEDs, red, green, blue and yellow (RGBY) LEDs, white and red (WR)LEDs, and RGBW LEDs.

The present invention is generally directed to illumination deviceshaving a plurality of light emitting diodes (LEDs) and one or morephotodetectors. For the sake of simplicity, the term “LED” will be usedthroughout this disclosure to refer to a single LED, or a chain ofserially connected LEDs supplied with the same drive current. Accordingto one embodiment, the present invention provides improved methods forcalibrating and compensating individual LEDs within an LED illuminationdevice, so as to accurately maintain a desired luminous flux and adesired chromaticity for the illumination device over changes in drivecurrent, temperature and/or time.

Although not limited to such, the present invention is particularly wellsuited to illumination devices (i.e., multi-colored illuminationdevices) in which two or more different colors of LEDs are combined toproduce blended white or near-white light, since the outputcharacteristics of differently colored LEDs vary differently over drivecurrent, temperature and time. The present invention is alsoparticularly well suited to illumination devices (i.e., tunableillumination devices) that enable the target dimming level and/or thetarget chromaticity setting to be changed by adjusting the drivecurrents supplied to one or more of the LEDs, since changes in drivecurrent inherently affect the lumen output, color and temperature of theillumination device.

FIGS. 2-3 illustrate how the relative luminous flux of an individual LEDchanges over junction temperature for different colors of LEDs. As shownin FIGS. 2-3, the luminous flux output from all LEDs generally decreaseswith increasing temperature. For some colors (e.g., white, blue andgreen), the relationship between luminous flux and junction temperatureis relatively linear (see FIG. 2), while for other colors (e.g., red,orange and especially yellow) the relationship is significantlynon-linear (see, FIG. 3). The chromaticity of an LED also changes withtemperature, due to shifts in the dominant wavelength (for both phosphorconverted and non-phosphor converted LEDs) and changes in the phosphorefficiency (for phosphor converted LEDs). In general, the peak emissionwavelength of green LEDs tends to decrease with increasing temperature,while the peak emission wavelength of red and blue LEDs tends toincrease with increasing temperature. While the change in chromacity isrelatively linear with temperature for most colors, red and yellow LEDstend to exhibit a more significant non-linear change.

As LEDs age, the luminous flux output from both phosphor converted andnon-phosphor converted LEDs, and the chromaticity of phosphor convertedLEDs, also changes over time. Early on in life, the luminous flux caneither increase (get brighter) or decrease (get dimmer), while late inlife, the luminous flux generally decreases. As expected, the lumenoutput decreases faster over time when the LEDs are subjected to higherdrive currents and higher temperatures. As a phosphor converted LEDages, the phosphor becomes less efficient and the amount of blue lightthat passes through the phosphor increases. This decrease in phosphorefficiency causes the overall color produced by the phosphor convertedLED to appear “cooler” over time. Although the dominant wavelength andchromaticity of a non-phosphor converted LED does not change over time,the luminous flux decreases as the LED ages, which in effect causes thechromaticity of a multi-colored LED illumination device to change overtime.

When differently colored LEDs are combined within a multi-coloredillumination device, the color point of the resulting device oftenchanges significantly with variations in temperature and over time. Forexample, when red, green and blue LEDs are combined within a white lightillumination device, the color point of the device may appearincreasingly “cooler” as the temperature rises. This is because theluminous flux produced by the red LEDs decreases significantly astemperatures increase, while the luminous flux produced by the green andblue LEDs remains relatively stable (see, FIGS. 2-3).

To account for temperature and aging effects, some prior artillumination devices attempt to maintain a consistent lumen outputand/or a consistent chromaticity over temperature and time by measuringcharacteristics of the emission LEDs and increasing the drive currentsupplied to one or more of the emission LEDs. For example, some priorart illumination devices measure the temperature of the illuminationdevice (either directly through an ambient temperature sensor or heatsink measurement, or indirectly through a forward voltage measurement),and adjust the drive currents supplied to one or more of the emissionLEDs to account for temperature related changes in lumen output. Otherprior art illumination devices measure the lumen output from individualemission LEDs, and if the measured value differs from a target value,the drive currents supplied to the emission LED are increased to accountfor changes in luminous flux that occur over time.

However, changing the drive currents supplied to the emission LEDsinherently affects the luminous flux and the chromaticity produced bythe LED illumination device. FIGS. 4 and 5 illustrate the relationshipbetween luminous flux and drive current for different colors of LEDs(e.g., red, red-orange, white, blue and green LEDs). In general, theluminous flux increases with larger drive currents, and decreases withsmaller drive currents. However, the change in luminous flux with drivecurrent is non-linear for all colors of LEDs, and this non-linearrelationship is substantially more pronounced for certain colors of LEDs(e.g., blue and green LEDs) than others. The chromaticity of theillumination also changes when drive currents are increased to combattemperature and/or aging effects, since larger drive currents inherentlyresult in higher LED junction temperatures (see, FIGS. 2-3). While thechange in chromaticity with drive current/temperature is relativelylinear for all colors of LEDs, the rate of change is different fordifferent LED colors and even from part to part.

Although some prior art illumination devices may adjust the drivecurrents supplied to the emission LEDs, these devices fail to provideaccurate temperature and age compensation by failing to account for thenon-linear relationship that exists between luminous flux and junctiontemperature for certain colors of LEDs (FIGS. 2-3), the non-linearrelationship that exists between luminous flux and drive current for allcolors of LEDs (FIGS. 4-5), and the fact that these relationships differfor different colors of LEDs. These devices also fail to account for thefact that the rate of change in chromaticity with drivecurrent/temperature is different for different colors of LEDs. Withoutaccounting for these behaviors, prior art illumination devices cannotprovide accurate temperature and age compensation for all LEDs includedwithin a multi-colored LED illumination device.

A need remains for improved illumination devices and methods forcalibrating and compensating individual LEDs included within anillumination device, so as to maintain a desired luminous flux and adesired chromaticity over variations in drive current and temperatureand over time, as the LEDs age. This need is particularly relevant tomulti-colored LED illumination devices, since different LED colorsrespond differently over temperature and time, and to illuminationdevices that provide dimming and/or color tuning capabilities, sincechanges in drive current inherently affect the lumen output, color andtemperature of the illumination device.

In order to meet these needs, improved illumination devices and methodsare provided herein to individually calibrate and compensate each LEDused in the LED illumination device. The improved calibration andcompensation methods described herein overcome the disadvantages ofconventional methods, which fail to provide accurate temperature and agecompensation for all LEDs included within an LED illumination device.

Exemplary Embodiments of Improved Methods for Calibrating anIllumination Device

FIG. 6 illustrates one embodiment of an improved method for calibratingan illumination device comprising a plurality of LEDs and at least onededicated photodetector. In some embodiments, the calibration methodshown in FIG. 6 may be used to calibrate an illumination device havingLEDs all of the same color. However, the calibration method describedherein is particularly well-suited for calibrating an illuminationdevice comprising two or more differently colored LEDs (i.e., amulti-colored LED illumination device), since output characteristics ofdifferently colored LEDs vary differently over drive current,temperature and time. The calibration method described herein is alsoparticularly well-suited for calibrating an illumination device thatprovides dimming and/or color tuning capabilities (i.e., a tunable LEDillumination device), since changes in drive current inherently affectthe lumen output, color and temperature of the illumination device.

Exemplary embodiments of an improved illumination device will bedescribed below with reference to FIGS. 16-20, which show differenttypes of LED illumination devices, each having one or more emittermodules. As described below, each emitter module may generally include aplurality of emission LEDs arranged in an array, and at least onededicated photodetector spaced about a periphery of the array. In oneexemplary embodiment, the array of emission LEDs may include red, green,blue and white (or yellow) LEDs, and the at least one dedicatedphotodetector may include one or more red, orange, yellow and/or greenLEDs. However, the present invention is not limited to any particularcolor, number, combination or arrangement of emission LEDs orphotodetectors. A skilled artisan would understand how the method stepsdescribed herein may be applied to other LED illumination devices havingsubstantially different emitter modules.

As shown in FIG. 6, the improved calibration method may generally beginby subjecting the illumination device to a first ambient temperature (instep 10). Once subjected to this temperature, a plurality of differentdrive current levels may be applied to the emission LEDs (in step 12)and a plurality of measurements may be obtained from both the emissionLEDs and the dedicated photodetector LED(s) at each of the differentdrive current levels (in steps 14 and 16). Specifically, two or moredifferent drive current levels may be successively applied to eachemission LED, one LED at a time, for the purpose of obtainingmeasurements from the illumination device. These measurements maygenerally include optical measurements and electrical measurements.

For example, a plurality of optical measurements may be obtained fromthe illumination produced by each emission LED at each of the differentdrive current levels (in step 14). According to one embodiment, theoptical measurements may include a plurality of luminous flux, xchromaticity and y chromaticity measurements, which are obtained foreach emission LED at two or more different drive current levels.However, the optical measurements described herein are not limited toluminous flux, x chromaticity and y chromaticity, and may includeadditional or alternative optical measurements in other embodiments ofthe invention.

In general, the chromaticity calibration values described herein maycorrespond to the CIE 1931 XYZ color space, the CIE 1931 RGB colorspace, the CIE 1976 LUV color space, and various other RGB color spaces(e.g., sRGB, Adobe RGB, etc.). Although the calibration and compensationmethods described herein acquire and utilize only x and y chromaticitycalibration values, one skilled in the art would understand howchromaticity values from other color spaces could be alternativelyacquired and used in the methods described herein. As such, thecalibration and compensation methods described herein and recited in theclaims are considered to encompass chromaticity calibration values fromany color space that can be used to describe the gamut of an LEDillumination device comprising substantially any combination of emissionLEDs as described herein.

In one preferred embodiment, three luminous flux (Luma) measurements,three x chromaticity (x chrom) measurements, and three y chromaticity (ychrom) measurements are measured from each emission LED at roughly amaximum drive current level (typically about 500 mA, depending on LEDpart number and manufacturer), roughly 30% of the maximum drive current,and roughly 10% of the maximum drive current, as shown in FIG. 7 anddiscussed below. In some embodiments, the luminous flux and x, ychromaticity measurements may be obtained from the emission LEDs usingan external calibration tool, such as a spectrophotometer. In someembodiments, the measurement values obtained from the externalcalibration tool may be transmitted wirelessly to the illuminationdevice, as described in more detail below with respect to FIG. 19.

In addition, a plurality of electrical measurements may be obtained fromeach of the emission LEDs and each of the dedicated photodetector(s) ateach of the different drive current levels (in step 16). Theseelectrical measurements may include, but are not limited to,photocurrents induced on the dedicated photodetector(s) and forwardvoltages measured across the dedicated photodetector(s) and/or theemission LEDs. Unlike the optical measurements described above, theelectrical measurements may be obtained from the dedicatedphotodetector(s) and the emission LEDs using the LED driver and receivercircuit included within the illumination device. An exemplary embodimentof such a circuit is shown in FIGS. 19-20 and described in more detailbelow.

At each of the different drive currents levels, the LED driver andreceiver circuit measures the photocurrents that are induced on thededicated photodetector by the illumination individually produced byeach emission LED. In one preferred embodiment, three photocurrent(Iph_d1) measurements may be obtained from the dedicated photodetectorfor each emission LED when the emission LEDs are successively driven toproduce illumination at three different drive current levels (e.g.,100%, 30% and 10% of a max drive level). In some embodiments, the LEDdriver and receiver circuit may obtain the photocurrent (Iph_d1)measurements at substantially the same time the external calibrationtool is measuring the luminous flux and x and y chromaticity of theillumination produced by the emission LEDs at each of the differentdrive current levels.

In general, the drive currents applied to the emission LEDs to measureluminous flux, chromaticity and induced photocurrent may be operativedrive current levels (e.g., about 20 mA to about 500 mA). In some cases,increasingly greater drive current levels may be successively applied toeach of the emission LEDs to obtain the measurements described herein.In other cases, the measurements may be obtained upon successivelyapplying decreasing levels of drive current to the emission LEDs. Theorder in which the drive current levels are applied is largelyunimportant, only that the drive currents be different from one another.

Although examples are provided herein, the present invention is notlimited to any particular value or any particular number of drivecurrent levels, and may apply substantially any value and any number ofdrive current levels to an emission LED within the operating currentlevel range of that LED. However, it is generally desired to obtain theluminous flux and chromaticity measurements from the emission LEDs andthe photocurrent measurements from the photodetector at a sufficientnumber of different drive current levels, so that the non-linearrelationship between these measurements and drive current can beaccurately characterized across the operating current level range of theLED.

While increasing the number of measurements does improve the accuracywith which the non-linear relationships are characterized, it alsoincreases the calibration time and costs. While the increase incalibration time and cost may not be warranted in all cases, it may bebeneficial in some. For example, additional luminous flux measurementsmay be beneficial when attempting to characterize the luminous flux vs.drive current relationship for certain colors of LEDs (e.g., blue andgreen LEDs), which tend to exhibit a significantly more non-linearrelationship than other colors of LEDs (see, FIGS. 4-5). Thus, a balanceshould be struck between accuracy and calibration time/costs whenselecting a desired number of drive current levels with which to obtainmeasurements for a particular color of LED.

Since increasing drive currents affect the junction temperature of theemission LEDs, a forward voltage may be measured across each emissionLED and each photodetector immediately after each operative drivecurrent level is supplied to the emission LEDs (in step 16). For eachoperative drive current level, the forward voltages can be measuredacross each emission LED and each photodetector before or after thephotocurrent measurements for that operative drive current level areobtained. Unlike the optical measurements, however, relatively smalldrive currents are applied to the emission LEDs and the dedicatedphotodetector(s) to measure the forward voltages developed there across.

In one preferred embodiment, three forward voltage (Vfe) measurementsmay be obtained from each emission LED and three forward voltage (Vfd1)measurements may be obtained from each dedicated photodetector (in step16) immediately after each of the different drive current levels (e.g.,100%, 30% and 10% of a max drive level) is applied to the emission LEDsto measure the luminous flux, x chromaticity and y chromaticity. Theforward voltage (Vfe and Vfd1) measurements can be obtained before orafter the induced photocurrents (Iph_d1) are measured at each of thedifferent drive current levels. By measuring the forward voltage (Vfe)across each emission LED and the forward voltage (Vfd1) across eachdedicated photodetector immediately after each operative drive currentlevel is applied to the emission LEDs, the Vfe and Vfd1 measurements maybe used to provide a good indication of how the junction temperature ofthe emission LEDs and the dedicated photodetector change with changes indrive current.

When taking forward voltage measurements, a relatively small drivecurrent is supplied to each of the emission LEDs and each of thededicated photodetector LEDs, one LED at a time, so that a forwardvoltage (Vfe or Vfd1) developed across the anode and cathode of theindividual LEDs can be measured (in step 16). When taking thesemeasurements, all other emission LEDs in the illumination device arepreferably turned “off” to avoid inaccurate forward voltage measurements(since light from other emission LEDs would induce additionalphotocurrents in the LED being measured).

As used herein, a “relatively small drive current” may be broadlydefined as a non-operative drive current, or a drive current level whichis insufficient to produce significant illumination from the LED. MostLED device manufacturers, which use forward voltage measurements tocompensate for temperature variations, supply a relatively large drivecurrent to the LEDs (e.g., an operative drive current level sufficientto produce illumination from the LEDs) when taking forward voltagemeasurements. Unfortunately, forward voltages measured at operativedrive current levels tend to vary significantly over the lifetime of anLED. As an LED ages, the parasitic resistance within the junctionincreases, which in turn, causes the forward voltage measured atoperating current levels to increase over time, regardless oftemperature. For this reason, a relatively small (i.e., non-operative)drive current is used herein when obtaining forward voltage measurementsto limit the resistive portion of the forward voltage drop.

For some common types of emission LEDs with one square millimeter ofjunction area, the optimum drive current used herein to obtain forwardvoltage measurements from the emission LEDs may be roughly 0.1-10 mA,and more preferably may be about 0.3-3 mA. In one embodiment, theoptimum drive current level may be about 1 mA for obtaining forwardvoltage measurements from the emission LEDs. However, smaller/largerLEDs may use proportionally less/more current to keep the currentdensity roughly the same. In the embodiments that use a significantlysmaller LED as the dedicated photodetector, the optimum drive currentlevel for obtaining forward voltage measurements from a singlephotodetector may range between about 100 μA to about 300 μA. In oneembodiment, the optimum drive current level used for obtaining forwardvoltage measurements from a plurality of dedicated photodetectorsconnected in parallel may be about lmA. The relatively small,non-operative drive currents used to obtain forward voltage measurementsfrom the emission LEDs (e.g., about 0.3 mA to about 3 mA) and therelatively small, non-operative drive currents used to obtain forwardvoltage measurements from a dedicated photodetector (e.g., about 100 μAto about 300 μA) are substantially smaller than the operative drivecurrent levels (e.g., about 20 mA to about 500 mA) used in steps 14 and16 to measure luminous flux, chromaticity and induced photocurrent.

After the measurements described in steps 14-16 are obtained at thefirst temperature, the illumination device is subjected to a secondambient temperature, which is substantially different from the firstambient temperature (in step 18). Once subjected to this secondtemperature, steps 12-16 are repeated (in step 20) to obtain anadditional plurality of optical measurements from each of the emissionLEDs (in step 14), and an additional plurality of electricalmeasurements from the emission LEDs and the dedicated photodetector (instep 16). The additional measurements may be obtained at the secondambient temperature in the same manner described above for the firstambient temperature.

In one embodiment, the second ambient temperature may be substantiallyless than the first ambient temperature. For example, the second ambienttemperature may be approximately equal to room temperature (e.g.,roughly 25° C.), and the first ambient temperature may be substantiallygreater than room temperature. In one example, the first ambienttemperature may be closer to an elevated temperature (e.g., roughly 70°C.) or a maximum temperature (e.g., roughly 85° C.) at which the deviceis expected to operate. In an alternative embodiment, the second ambienttemperature may be substantially greater than the first ambienttemperature.

It is worth noting that the exact values, number and order in which thetemperatures are applied to calibrate the individual LEDs is somewhatunimportant. However, it is generally desired to obtain the luminousflux, x and y chromaticity, and photocurrent calibration values at anumber of different temperatures, so that the non-linear relationshipsbetween these measurements and drive current can be accuratelycharacterized across the operating temperature range of each LED. In onepreferred embodiment, the illumination device may be subjected to twosubstantially different ambient temperatures, which are selected fromacross the operating temperature range of the illumination device. Whileit is possible to obtain the measurements described herein at three (ormore) temperatures, doing so may add significant expense, complexityand/or time to the calibration process. For this reason, it is generallypreferred that the emission LEDs and the dedicated photodetector(s) becalibrated at only two different temperatures (e.g., about 25° C. andabout 70° C.).

In some embodiments, the illumination device may be subjected to thefirst and second ambient temperatures by artificially generating thetemperatures during the calibration process. However, it is generallypreferred that the first and second ambient temperatures are ones whichoccur naturally during production of the illumination device, as thissimplifies the calibration process and significantly decreases the costsassociated therewith. In one embodiment, the measurements obtained atthe elevated temperature may be taken after burn-in of the LEDs when theillumination device is relatively hot (e.g., roughly 50° C. to 85° C.),and sometime thereafter (e.g., at the end of the manufacturing line), aroom temperature calibration may be performed to obtain measurementswhen the illumination device is relatively cool (e.g., roughly 20° C. to30° C.).

Once the calibration measurements are obtained, the calibration valuesmay be stored within the illumination device (in step 22), so that thestored values can be later used to compensate the illumination devicefor changes in luminous flux and/or chromaticity that may occur overvariations in drive current, temperature and time. In one embodiment,the calibration values may be stored within a table of calibrationvalues as shown, for example, in FIG. 7. The table of calibration valuesmay be stored within a storage medium of the illumination device, asdiscussed below with reference to FIG. 19.

FIG. 7 illustrates one embodiment of a calibration table that may begenerated in accordance with the calibration method shown in FIG. 6. Inthe illustrated embodiment, the calibration table includes six luminousflux measurements (Luma), six x chromaticity measurements (x chrom), andsix y chromaticity measurements (y chrom), which were obtained from eachemission LED (e.g., white, blue, green and red emission LEDs) at thethree different drive currents (e.g., 10%, 30% and 100% of a max drivecurrent) and the two different temperatures (T0, T1) in steps 10, 12,14, 18, 20 and 22 of the calibration method. The calibration table shownin FIG. 7 also includes six photocurrent measurements (Iph_d1) that wereinduced on the photodetector by the illumination produced by each of theemission LEDs at the three different drive currents levels and the twodifferent temperatures in steps 10, 12, 16, 20 and 22 of the calibrationmethod.

For each emission LED (e.g., each white, blue, green and red emissionLED) and each ambient temperature (T0, T1), the calibration table shownin FIG. 7 also includes the forward voltage (Vfe) that was measuredacross the emission LED and the forward voltage (Vfd1) that was measuredacross the dedicated photodetector immediately after each of the threedifferent drive currents levels is supplied to the emission LEDs. Inthis example embodiment, steps 10, 12, 16, 18, 20 and 22 of thecalibration method result in six Vfe measurements and six Vfd1measurements being stored for each emission LED, as shown in FIG. 7.

The calibration table shown in FIG. 7 represents only one example of thecalibration values that may be stored within an LED illumination device,in accordance with the calibration method described herein. In someembodiments, the calibration method shown in FIG. 6 may be used to storesubstantially different calibration values, or substantially differentnumbers of calibration values, within the calibration table of the LEDillumination device.

As noted above, the present invention is not limited to the exemplarynumber of drive current levels and values of drive current shown inFIGS. 6 and 7. It is certainly possible to obtain a greater/lessernumber of optical and electrical measurements from the emission LEDs andthe at least one dedicated photodetector by applying a greater/lessernumber of drive current levels to the emission LEDs. It is also possibleto use substantially different values of drive current, other than the10%, 30% and 100% of the max drive current illustrated in FIG. 7.

It is also possible to obtain and store a different number of forwardvoltage (Vfe) measurements from the emission LEDs, or a different numberof forward voltage (Vfd1) measurements from the at least one dedicatedphotodetector. For example, the calibration table shown in theembodiment of FIG. 7 stores six forward voltage (Vfe) measurements fromeach emission LED and six*n forward voltage (Vfd1) measurements fromeach dedicated photodetector, where ‘n’ is the number of emission LEDsincluded within the illumination device. As noted above, the six Vfemeasurements and six*n Vfd measurements are preferably obtained at twodifferent ambient temperatures (T0, T1) immediately after each operativedrive current level (e.g., 10%, 30% and 100% of a max drive current) isapplied to each emission LED. Such an embodiment is generally preferred,as it provides a good indication of how the emitter and detectorjunction temperatures change with changes in ambient temperature andchanges in drive current. In addition, such an embodiment enables thecompensation method shown in FIG. 13 (and described below) to compensatefor emitter aging when only detector forward voltages (Vfd1) aremeasured during operation of the device.

As shown in FIGS. 6-7 and described above, the calibration method mayobtain only one Vfe and only one Vfd1 measurement for each emission LEDat a given temperature (e.g., T0) and a given drive current (e.g., 10%of the max drive current). In one alternative embodiment, thecalibration method of FIG. 6 may obtain a plurality of Vfe and aplurality of Vfd1 measurements for each emission LED at a giventemperature (e.g., T0) and a given drive current (e.g., 10% of the maxdrive current). The plurality of Vfe and Vfd1 measurements may beobtained over a short period of time (e.g., 100 msec), and the pluralityof Vfe measurements and the plurality of Vfd1 measurements obtainedduring each time period may be averaged and filtered before they arestored within the calibration table of FIG. 7.

In another alternative embodiment, the calibration method of FIG. 6 mayobtain only two forward voltage (Vfe) measurements from each emissionLED, one for each of the two different temperatures (T0, T1), asdescribed in commonly assigned U.S. patent application Ser. Nos.13/970,944, 13/970,964 and 13/970,990. Likewise, only two*n forwardvoltage (Vfd1) measurements may be obtained from the dedicatedphotodetector, where ‘n’ is the number of emission LEDs included withinthe illumination device. In this embodiment, however, the forwardvoltage (Vfe and Vfd1) measurements stored in the calibration tablewould only provide an indication of how the emitter and detectorjunction temperatures change with changes in ambient temperature, notwith drive current induced temperature changes.

In another alternative embodiment of the invention, the calibrationmethod shown in FIG. 6 may omit the emitter forward voltage (Vfe)measurements altogether, and rely solely on the photodetector forwardvoltage (Vfd1) measurements to provide an indication of temperature.However, the Vfe measurements may only be omitted if the temperaturedifference between the emission LEDs and the dedicated photodetector(s)remains relatively the same over the operating temperature range. Tomaintain a consistent temperature difference between the emission LEDsand the photodetector(s), an improved emitter module is provided hereinand described below with reference to FIG. 18A.

In yet another alternative embodiment of the invention, the calibrationmethod shown in FIG. 6 may be used to obtain additional measurements,which may be later used to compensate for phosphor aging, and thereby,control the chromaticity of a phosphor converted white LED over time.

As noted above, some embodiments of the invention may include a phosphorconverted white emission LED within the emitter module. These LEDs maybe formed by coating or covering, e.g., a blue LED having a peakemission wavelength of about 400-500 with a phosphor material (e.g.,YAG) to produce substantially white light with a CCT of about 3000K.Other combinations of LEDs and phosphors may be used to form a phosphorconverted LED, which is capable of producing white or near-white lightwith a CCT in the range of about 2700K to about 10,000 k.

In phosphor converted LEDs, the spectral content of the LED combineswith the spectral content of the phosphor to produce white or near-whitelight. As shown in FIG. 21, the combined spectrum may include a firstportion having a first peak emission wavelength (e.g., about 400-500),and a second portion having a second peak emission wavelength (e.g.,about 500-650), which is substantially different from the first peakemission wavelength. In this example, the first portion of the spectrumis generated by the light emitted by the blue LED, and the secondportion is generated by the light that passes through the phosphor(e.g., YAG).

As the phosphor converted LED ages, the efficiency of the phosphordecreases, which causes the chromaticity of the phosphor converted LEDto appear “cooler” over time. In order to account for age-relatedchromaticity shifts in a phosphor converted LED, it may be desirable insome embodiments of the calibration method shown in FIG. 6 to measurethe photocurrents induced by the LED portion and the photocurrentsinduced by the phosphor portion of the phosphor converted LEDseparately. Thus, some embodiments of the invention may use twodifferent colors of photodetectors to measure photocurrents, which areseparately induced by different portions of the phosphor converted LEDspectrum. In particular, an emitter module of the illumination devicemay include a first photodetector whose detection range is configuredfor detecting only the first portion of the spectrum emitted by thephosphor converted LED, and a second photodetector whose detection rangeis configured for detecting only the second portion of the spectrumemitted by the phosphor converted LED.

In general, the detection range of the first and second photodetectorsmay be selected based on the spectrum of the phosphor converted LEDbeing measured. In the exemplary embodiment described above, in which aphosphor converted white emission LED is included within the emittermodule and implemented as described above, the detection range of thefirst photodetector may range between about 400 nm and about 500 nm formeasuring the photocurrents induced by light emitted by the blue LEDportion, and the detection range of the second photodetector may rangebetween about 500 nm and about 650 nm for measuring the photocurrentsinduced by light that passes through the phosphor portion of thephosphor converted white LED. The first and second photodetectors mayinclude dedicated photodetectors and/or emission LEDs, which aresometimes configured for detecting incident light.

As noted above, the emitter module of the illumination device preferablyincludes at least one dedicated photodetector. In one embodiment, theemitter module may include two different colors of dedicatedphotodetectors, such as one or more dedicated green photodetectors andone or more dedicated red photodetectors (see, e.g., FIG. 17B). Inanother embodiment, the emitter module may include only one dedicatedphotodetector, such as a single red, orange or yellow photodetector(see, e.g., FIG. 16B). In such an embodiment, one of the emission LEDs(e.g., the green emission LED) may be configured, at times, as aphotodetector for measuring a portion of the phosphor converted LEDspectrum.

In the calibration method described above and shown in FIG. 6, the atleast one dedicated photodetector may be used in step 16 to measure thephotocurrents (Iph_d1), which are induced in the dedicated photodetectorby the illumination produced by each of the emission LEDs when theemission LEDs are successively driven to produce illumination at theplurality of different drive current levels (e.g., 100%, 30% and 10% ofa max drive level) and the plurality of different temperatures (e.g., T0and T1). Sometime before or after each of the photocurrent measurements(Iph_d1) is obtained from the dedicated photodetector, a forward voltage(Vfd1) is measured across the dedicated photodetector to provide anindication of the detector junction temperature at each of thecalibrated drive current levels.

In some embodiments of the calibration method shown in FIG. 6, thededicated photodetector used to obtain the photocurrent (Iph_d1) andforward voltage (Vfd1) measurements may be, e.g., a red LED. Whencalibrating a phosphor converted white LED, the dedicated redphotodetector may be used to measure the photocurrent (Iph_d1) inducedby the light that passes through the phosphor (i.e., the “secondportion” of the spectrum shown in FIG. 21). In some embodiments, anotherdedicated photodetector (or one of the emission LEDs) may be used tomeasure the photocurrent (Iph_d2), which is induced by the light emittedby the LED portion (i.e., the “first portion” of the spectrum shown inFIG. 21) of the phosphor converted white LED. This photodetector may be,for example, a dedicated green photodetector or one of the greenemission LEDs.

As shown in FIG. 7, the additional photodetector may be used in step 16of the calibration method shown in FIG. 6 to measure the photocurrents(Iph_d2), which are induced in the additional photodetector by theillumination produced by the LED portion of the phosphor converted whiteLED when that LED is successively driven to produce illumination at aplurality of different drive current levels (e.g., 100%, 30% and 10% ofa max drive level) and a plurality of different temperatures (e.g., T0and T1). In addition to measuring the photocurrents induced by the LEDportion of the phosphor converted white LED, the photocurrents (Iph_d2)induced by the illumination produced by the blue emission LED may alsobe obtained from the additional photodetector in step 16 and used as areference in the compensation method of FIG. 13. Before or after each ofthe photocurrent measurements (Iph_d2) is obtained from the additionalphotodetector, a forward voltage (Vfd2) is measured across theadditional photodetector to provide an indication of the detectorjunction temperature at each of the calibrated drive current levels.

In addition to storing separate photocurrent measurements (Iph_d2 andIph_d1) for the phosphor converted white LED, the calibration table mayalso store separate luminous flux (Luma), x chromaticity (x chrom) and ychromaticity (y chrom) measurements for the LED portion and the phosphorportion of the phosphor converted white LED spectrum at each of thecalibrated drive currents and temperatures. While this is not explicitlyshown in FIG. 7, measuring the luminous flux (Luma), x chromaticity (xchrom) and y chromaticity (y chrom) attributed to each portion of thephosphor converted white LED spectrum, and storing these values withinthe calibration table, the stored calibration values may be later usedduring one or more of the compensation methods described herein tocontrol the luminous flux and chromaticity of the LED portion and thephosphor portion of the phosphor converted white LED, separately, as ifthe LED were two different LEDs.

Exemplary methods for calibrating an illumination device comprising aplurality of emission LEDs and one or more photodetectors has now beendescribed with reference to FIGS. 6-7. Although the method steps shownin FIG. 6 are described as occurring in a particular order, one or moreof the steps of the illustrated method may be performed in asubstantially different order. In one alternative embodiment, forexample, the plurality of electrical measurements (e.g., Iph, Vfd andVfe) may be obtained from the one or more photodetector(s) and emissionLEDs in step 16 before the plurality of optical measurements (e.g.,Luma, x chrom, y chrom) are obtained from the emission LEDs in step 14.In another alternative embodiment, the external calibration tool mayobtain the optical measurements (in step 14) at substantially the sametime as the LED driver and receiver circuit is obtaining the electricalmeasurements (in step 16). While the calibration method shown in FIG. 6stores the calibration values within the illumination device at the endof the calibration method (e.g., in step 22), a skilled artisan wouldrecognize that these values may be stored at substantially any timeduring the calibration process without departing from the scope of theinvention. The calibration method described herein is considered toencompass all such variations and alternative embodiments.

The calibration method provided herein improves upon conventionalcalibration methods in a number of ways. First, the method describedherein calibrates each emission LED (or chain of LEDs) individually,while turning off all other emission LEDs not currently under test. Thisnot only improves the accuracy of the stored calibration values, butalso enables the stored calibration values to account for processvariations between individual LEDs, as well as differences in outputcharacteristics that inherently occur between different colors of LEDs.

Accuracy is further improved herein by supplying a relatively small(i.e., non-operative) drive current to the emission LEDs and thephotodetector(s) when obtaining forward voltage measurements, as opposedto the operative drive current levels typically used in conventionalcalibration methods. By using non-operative drive currents to obtain theforward voltage measurements, the present invention avoids inaccuratecompensation by ensuring that the forward voltage measurements for agiven temperature and fixed drive current do not change significantlyover time (due to parasitic resistances in the junction when operativedrive currents are used to obtain forward voltage measurements).

As another advantage, the calibration method described herein obtains aplurality of optical measurements from each emission LED and a pluralityof electrical measurements from each photodetector at a plurality ofdifferent drive current levels and a plurality of differenttemperatures. This further improves calibration accuracy by enabling thenon-linear relationship between luminous flux and drive current and thenon-linear relationship between photocurrent and drive current to beprecisely characterized for each individual LED. Furthermore, obtainingthe calibration values at a number of different ambient temperaturesimproves compensation accuracy by enabling the compensation methods(described below) to interpolate between the stored calibration values,so that accurate compensation values may be determined for currentoperating temperatures.

As yet another advantage, the calibration method described herein mayuse different colors of photodetectors to measure photocurrents, whichare induced by different portions (e.g., an LED portion and a phosphorportion) of a phosphor converted LED spectrum. The different colors ofphotodetectors may also be used to measure the photocurrent, which isinduced by a reference emission LED, whose peak emission wavelengthfalls within the LED portion of the spectrum emitted by the phosphorconverted LED. By storing these calibration values within theillumination device, the calibration values may be later used to detectand account for chromaticity shifts that may occur in a phosphorconverted LED over time.

As described in more detail below, the calibration values stored withinthe calibration table can be used in one or more compensation methodsdescribed herein to adjust the individual drive currents supplied to theemission LEDs, so as to obtain a desired luminous flux and a desiredchromaticity over changes in drive current, changes in temperature andover time, as the LEDs age. For example, the luminous flux (Luma)measurements may be used in some embodiments to maintain a consistentlumen output and chromaticity over changes in temperature. In otherembodiments, the luminous flux (Luma) measurements may be used alongwith the chromaticity (e.g., x chrom, y chrom) measurements to obtain anew target lumen output or a new target chromaticity when the dimminglevel or color point setting for the illumination device is changed.Regardless of the particular luminous flux or chromaticity setting used,the photocurrent (Iph) measurements may be used to adjust the individualdrive currents supplied to the emission LEDs to account for LED agingeffects. While the most accurate results may be obtained by utilizingall such measurements when compensating an LED illumination device, oneskilled in the art would understand how one or more of the calibrationvalues described herein may be used to improve upon the compensationmethods performed by prior art illumination devices.

Exemplary Embodiments of Improved Methods for Controlling anIllumination Device

FIGS. 8-15 illustrate exemplary embodiments of improved methods forcontrolling an illumination device that generally includes a pluralityof emission LEDs and at least one dedicated photodetector. Morespecifically, FIGS. 8-15 illustrate exemplary embodiments of improvedcompensation methods that may be used to adjust the drive currentssupplied to individual LEDs of an LED illumination device, so as toobtain a desired luminous flux and a desired chromaticity over changesin drive current, changes in temperature and over time, as the LEDs age.

In some embodiments, the compensation methods shown in FIGS. 8-15 may beused to control an illumination device having LEDs all of the samecolor. However, the compensation methods described herein areparticularly well-suited for controlling an illumination devicecomprising two or more differently colored LEDs (i.e., a multi-coloredLED illumination device), since output characteristics of differentlycolored LEDs vary differently over drive current, temperature and time.The compensation methods described herein are also particularlywell-suited for controlling an illumination device that provides dimmingand/or color tuning capabilities (i.e., a tunable LED illuminationdevice), since changes in drive current inherently affect the lumenoutput, color and temperature of the illumination device.

Exemplary embodiments of an illumination device will be described belowwith reference to FIGS. 16-20, which show different types of LEDillumination devices, each having one or more emitter modules. As shownin FIGS. 16B, 17B, 18A and 18B, each emitter module may generallyinclude a plurality of emission LEDs arranged in an array, and one ormore photodetectors spaced about a periphery of the array. In oneexemplary embodiment, the array of emission LEDs may include red, green,blue and white (or yellow) LEDs, and the one or more photodetectors mayinclude one or more red, orange, yellow and/or green LEDs. However, thepresent invention is not limited to any particular color, number,combination or arrangement of emission LEDs and photodetectors. Askilled artisan would understand how the method steps described hereinmay be applied to other LED illumination devices having substantiallydifferent emitter modules.

According to one embodiment, FIG. 8 illustrates an exemplarycompensation method that may be used to adjust the drive currentssupplied to individual LEDs of an LED illumination device, so as tomaintain a desired luminous flux and a desired chromaticity over changesin temperature. The compensation method steps shown in solid outline inFIG. 8 are similar to the compensation method steps described in FIG. 5of commonly assigned U.S. patent application Ser. Nos. 13/970,944,13/970,964 and 13/970,990, which provided improved illumination devicesand temperature compensation methods.

However, the previously filed applications failed to disclose methodsfor controlling an illumination device, so as to maintain a desiredluminous flux and a desired chromaticity over time, or to obtain a newluminous flux or a new chromaticity when the dimming level or the colorpoint setting is changed for the illumination device. The optionalmethod steps shown in dashed outline in FIG. 8 improve upon thecompensation method described in the previously filed applications byenabling the luminous flux and chromaticity to be precisely controlledover a range of dimming levels and a range of color point settings. Thecompensation method shown in FIG. 13, and discussed below, furtherimproves upon the compensation method described in the previously filedapplications by providing a method for maintaining a desired luminousflux and a desired chromaticity over time, as the LEDs age. Thecompensation methods shown in FIGS. 8 and 13 may be used alone, ortogether to provide accurate compensation for all LEDs used in theillumination device over changes in drive current, temperature and time.

As shown in FIG. 8, the improved compensation method may generally beginby driving the plurality of emission LEDs substantially continuously toproduce illumination (in step 30). When the illumination device is firstturned “on,” the ambient temperature surrounding the LEDs steadilyincreases over time, until the temperature stabilizes. In someembodiments, the compensation method may be performed soon after theillumination device is turned “on,” and may be repeated a number oftimes until the temperature surrounding the LEDs stabilizes. However,since changes in drive current and other factors affect the ambienttemperature surrounding the LEDs, the compensation method shown in FIG.8 may be performed at other times during operation of the illuminationdevice.

In one preferred embodiment, the compensation method may be performedonly if a significant change in ambient temperature is detected (in step32). In some embodiments, the ambient temperature may be monitored by adedicated temperature sensor or an additional LED, which is used as botha temperature sensor and an optical sensor, as discussed below withreference to FIG. 20. A “significant change” may be any incrementalincrease or decrease in ambient temperature (e.g., 1° C.) and isgenerally set within the control circuitry of the illumination device asa specified amount. If no significant temperature change is detected(“No” branch in step 32), the compensation method may continue to drivethe plurality of emission LEDs substantially continuously to produceillumination (in step 30). Otherwise, the compensation method mayproceed to step 36.

In some embodiments, the compensation method may also be performed ifthe dimming level and/or the color point setting is changed in a tunableillumination device. To accommodate such changes, the compensationmethod may monitor the target luminance (Ym) and the target chromaticitysetting (xm, ym) stored within the control circuitry of the illuminationdevice. In some embodiments, the target luminance (Ym) may be expressedas a finite set of dimming levels, and the target chromaticity setting(xm, ym) may be expressed as a range of x,y coordinates within the colorgamut of the illumination device. In some embodiments, the targetluminance (Ym) and the target chromaticity setting (xm, ym) may bestored, e.g., as 16 bit integer values within a hardware or softwareregister of the control circuitry, and may be changed by a user or abuilding controller by storing alternative values within theillumination device. If no changes to the stored Ym, xm and ym valuesare detected by the control circuitry (“No” branch in step 34), thecompensation method may continue to drive the plurality of emission LEDssubstantially continuously to produce illumination (in step 30).Otherwise, the compensation method may proceed to step 36.

As used herein, the term “substantially continuously” means that anoperative drive current is supplied to the plurality of emission LEDsalmost continuously, with the exception of periodic intervals duringwhich the plurality of emission LEDs are momentarily turned “off” forshort durations of time (in step 36). In the exemplary embodiment ofFIG. 8, the periodic intervals are utilized for obtaining forwardvoltage (Vfe) measurements from each of the emission LEDs, one LED at atime (in step 38). These periodic intervals may also be used for otherpurposes, as shown in FIGS. 13-14 and discussed in more detail below.

FIG. 9 is an exemplary timing diagram illustrating steps 30, 36 and 38of the compensation method shown in FIG. 8, according to one embodimentof the invention. As shown in FIGS. 8 and 9, the plurality of emissionLEDS are driven substantially continuously with operative drive currentlevels (denoted generically as I1 in FIG. 9) to produce illumination (instep 30 of FIG. 8). At periodic intervals, the plurality of emissionLEDs are turned off for short durations of time (in step 36 of FIG. 8)by removing the drive currents, or at least reducing the drive currentsto non-operative levels (denoted generically as I0 in FIG. 9). Betweenthe periodic intervals, the illumination device produces continuousillumination with DC current supplied to the LEDs.

During each periodic interval, one emission LED is driven with arelatively small, non-operative drive current (e.g., approximately0.1-10 mA, not shown in FIG. 9) and the forward voltage developed acrossthat LED is measured (e.g., Vfe1). In this manner, forward voltages(e.g., Vfe1, Vfe2, Vfe3 and Vfe4) are measured across each emission LED,one LED at a time, as shown in FIG. 9. In some cases, the forwardvoltages associated with each individual emission LED may be averagedover a period of time, filtered to eliminate erroneous data, and storedfor example in a register of the illumination device.

FIG. 9 provides an exemplary timing diagram for measuring the presentforward voltages (i.e., Vfe_present, or the forward voltages thatdevelop across the emission LEDs during operation of the device), whichprovide an indication of the current junction temperatures of eachemission LED included in the illumination device. Although FIG. 9provides an exemplary timing diagram for an illumination devicecomprising four emission LEDs, such as RGBY or RGBW, the timing diagramand method described herein can be easily modified to accommodate afewer or greater number of emission LEDs. As described in more detailbelow, the timing diagram described herein can also be easily modifiedto accommodate additional measurements, as shown for example in FIG. 14.

In one alternative embodiment, the compensation method of FIG. 8 maymeasure the detector forward voltage (Vfd_present) that develops acrossthe one or more photodetectors in step 38, instead of the emitterforward voltages (Vfe_present) shown in FIGS. 8-9. However, the detectorforward voltage may only be used in the presently described compensationmethod if a temperature difference between the emission LEDs and thephotodetector(s) remains relatively the same over the operatingtemperature range. To maintain a substantially consistent temperaturedifference between the emission LEDs and the photodetector(s), animproved emitter module is provided herein and described below withreference to FIG. 18A.

In some embodiments of the invention, the compensation method shown inFIG. 8 may continue to step 46 if a temperature change was detected instep 32, but the target luminance (Ym) and the target chromaticitysetting (xm, ym) were not changed in step 34. As shown in FIGS. 8 and10, the compensation method may determine the drive current (Ix) neededto achieve a desired luminous flux (Lx) from each emission LED using thepresent forward voltages (Vfe_present), the table of stored calibrationvalues generated during the calibration method of FIG. 6 and one or moreinterpolation techniques (in step 46 of FIG. 8). For example, thecompensation method may interpolate between the stored luminous fluxcalibration values to calculate a luminous flux value that would beexpected for the present forward voltage at each of the previouslycalibrated (i.e., known) drive current levels. Once a luminous fluxvalue is calculated at each of the calibrated drive current levels,another interpolation technique may be used to determine an unknowndrive current (Ix) needed to produce a desired luminous flux (Lx),should the desired luminous flux differ from one of the calculatedluminous flux values.

FIG. 10 is a graphical illustration depicting how one or moreinterpolation technique(s) may be used to determine the drive current(Ix) needed to produce a desired luminous flux (Lx) from the storedcalibration values. In FIG. 10, the six solid dots (●'s) represent theluminous flux calibration values, which were obtained during calibrationat three different drive currents (e.g., 10%, 30% and 100% of themaximum drive current) and two different temperatures (e.g., T0 and T1).As noted above, the luminous flux calibration values (●'s) werepreviously stored within a table of calibration values (see, e.g., FIG.7) for each emission LED included in the illumination device.

To determine the drive current (Ix) needed to produce a desired luminousflux (Lx) for a given LED, the compensation method of FIG. 8interpolates between each of the stored calibration values (●'s) tocalculate the luminous flux values (Δ's), which should be produced atthe present operating temperature (Vfe_present) when using the samethree drive currents (e.g., 10%, 30% and 100% of the maximum drivecurrent). If the desired luminous flux (Lx) differs from one of thecalculated luminous flux values (Δ's), the compensation method of FIG. 8may apply another interpolation technique to the calculated luminousflux values (Δ's) to generate a relationship there between (denoted by asolid line in FIG. 10). From this relationship, the drive current (Ix)needed to produce a desired luminous flux (Lx) may be determined.

In some embodiments, the interpolation technique(s) used to calculatethe luminous flux values (Δ's) and to generate the relationship betweenthe calculated values may depend on the color of the LED beingcompensated. For example, the luminous flux vs. junction temperature (orforward voltage) relationship for blue, green and white LEDs issubstantially linear over the operating temperature range (see, FIG. 2).Because of this linear relationship, the compensation method is able tocalculate luminous flux values (Δ's) at the present forward voltage forblue, green and white LEDs by linearly interpolating between thecalibration values stored at each of the calibrated drive currentlevels. However, red, red-orange and yellow LEDs exhibit a substantiallymore non-linear relationship between luminous flux vs. junctiontemperature (see, FIG. 3). For these LEDs, a higher-order interpolationtechnique may be used to calculate the luminous flux values (Δ's) at thepresent forward voltage for each of the calibrated drive current levels.

In one embodiment, the higher-order interpolation technique may be inthe form of a quadratic interpolation, which follows the generalequation:ax ² +bx+c=y  (EQ. 1)where ‘x’ is Vf (or temperature), ‘y’ is luminous flux, and ‘a,’ ‘b’ and‘c’ are coefficients. If forward voltage and luminous flux values werepreviously obtained during the calibration phase at three differenttemperatures, the ‘a,’ ‘b’ and ‘c’ coefficient values may be preciselydetermined by inserting the stored calibration values into EQ. 1 andseparately solving the equation for ‘a,’ ‘b’ and ‘c’. If, on the otherhand, the LED was calibrated at only two different temperatures, asdescribed above, the ‘a’ coefficient may be obtained from data sheetsprovided by the LED manufacturer, while the ‘b’ and ‘c’ coefficients aredetermined from the calibration values. While the latter method(sometimes referred to as a “poor man's quadratic interpolation”) maysacrifice a small amount of accuracy, it may represent an acceptabletrade-off between accuracy and calibration costs. In some embodiments,the calibration table may store the actual (i.e., measured) luminousflux calibration values, or just the coefficients needed for thehigher-order interpolation. In other embodiments, a combination ofmeasured values and coefficients may be stored in the calibration table.

In some embodiments, the relationship (solid line in FIG. 10) betweenthe calculated luminous flux values (Δ's) may be determined throughanother interpolation technique, if a desired luminous flux (Lx) differsfrom one of the calculated values. However, since the relationshipbetween luminous flux and drive current is non-linear for all LED colors(see, FIGS. 4-5), the relationship is preferably derived through ahigher-order interpolation of the calculated luminous flux values.Alternatively, a piece-wise linear interpolation could be used tocharacterize the non-linear relationship between the calculated luminousflux values, or a typical curvature could be assumed from data sheetsprovided by the LED manufacturer.

In some embodiments, an appropriate interpolation technique may beselected based on trade-offs between memory and processing requirements,and/or based upon the particular color of LED being compensated. Asnoted above, some LED colors, such as blue and green, exhibit acomparatively more non-linear luminous flux vs. drive currentrelationship than other LED colors, such as red and red-orange (see,FIGS. 4-5). LED colors exhibiting substantially greater non-linearbehaviors (such as blue and green) may be more accurately compensated byobtaining more luminous flux calibration values and using a piece-wiselinear interpolation technique, or by obtaining fewer calibration valuesand using a higher-order interpolation technique or an assumed curvatureto generate the non-linear relationship between luminous flux and drivecurrent.

Once the relationship between luminous flux and drive current is derivedfor a given LED, the drive current (Ix) needed to produce a desiredluminous flux (Lx) may be selected from the generated relationship, asshown in the example of FIG. 10. The selected drive current may then beused to drive the LED to produce illumination with the desired luminousflux (in step 48 of FIG. 8). This process is performed for each emissionLED, until each LED is configured for producing a desired luminous flux(Lx) at the present operating temperature (Vfe_present). The drivecurrents supplied to the LEDs may be adjusted to meet the selected drivecurrents either by adjusting the drive current level (i.e., currentdimming), or by changing the duty cycle of the drive current throughPulse Width Modulation (PWM) dimming.

In some embodiments, the compensation method shown in FIG. 8 may performadditional steps (e.g., steps 40, 42, and 44), if the target luminance(Ym) and/or the target chromaticity setting (xm, ym) was changed in step34. As noted above, the target luminance (Ym) or target chromaticitysetting (xm, ym) may be changed by a user or a building controller toadjust the dimming level or the color point setting of a tunable LEDillumination device. If a change in either target luminance orchromaticity is detected in step 34, the compensation method maydetermine the drive currents (Idrv), which are presently supplied toeach of the emission LEDs (in step 40). The present drive currents(Idrv) may be obtained from the LED driver circuitry. Althoughillustrated in FIG. 8 as occurring after the present forward voltages(Vfe_present) are measured in step 38, the present drive currents (Idrv)may be determined before the forward voltages are measured, inalternative embodiments of the invention.

Once the present forward voltages and the present drive currents aredetermined, the compensation method may determine the chromaticityvalues (x_(i), y_(i)) that are expected for each emission LED (in step42) using the present forward voltage (Vfe_present) measured across theLED, the present drive current (Idrv) supplied to the LED, the table ofstored calibration values generated during the calibration method ofFIG. 6, and one or more interpolation techniques. FIGS. 11-12 depict howone or more interpolation technique(s) may be used to determine theexpected x and y chromaticity values (x_(i), y_(i)) for a given LED atthe present operating temperature (Vfe_present) and the present drivecurrent (Idrv) from the table of stored calibration values.

In FIGS. 11-12, the solid dots (●'s) represent the x and y chromaticitycalibration values, which were obtained during calibration at threedifferent drive currents (e.g., 10%, 30% and 100% of the maximum drivecurrent) and two different temperatures (e.g., T0 and T1). As notedabove, these x and y chromaticity calibration values (●'s) were storedwithin a table of calibration values (see, e.g., FIG. 7) for eachemission LED included within the illumination device. To determine theexpected x and y chromaticity values (x_(i), y_(i)) for a given LED, thecompensation method of FIG. 8 interpolates between the storedcalibration values (●'s) to calculate the x and y chromaticity values(Δ's), which should be produced at the present operating temperature(Vfe_present) when using the same three drive currents (e.g., 10%, 30%,and 100% of the maximum drive current). In most cases, a linearinterpolation technique may be used to calculate the x and ychromaticity values (Δ's) at the present operating temperature(Vfe_present). However, if the x and y chromaticity values werecalibrated at more than two temperatures, a non-linear interpolationtechnique may be used to calculate the x and y chromaticity values (Δ's)at the present operating temperature (Vfe_present).

If the drive current (Idrv) presently supplied to the emission LEDdiffers from one of the calibrated drive current levels, thecompensation method of FIG. 8 may apply another interpolation techniqueto the calculated x and y chromaticity values (Δ's) to generate arelationship there between (denoted by a solid line in FIGS. 11-12).From this relationship, the expected x and y chromaticity values (x_(i),y_(i)) may be determined for the present drive current (Idrv).

The interpolation technique used to generate the relationship betweenthe calculated x and y chromaticity values (A's) may generally depend onthe color of LED being compensated. As noted above, the change inchromaticity with drive current and temperature is significantlydifferent for different colors of LEDs. For a red LED, the peak emissionwavelength increases relatively linearly with drive current, yetincreases significantly with temperature. The peak emission wavelengthof a blue LED decreases somewhat non-linearly with drive current andincreases slightly with temperature. Although the peak emissionwavelength of a green LED varies very little with temperature, itdecreases very non-linearly with drive current. In some embodiments, apiecewise linear interpolation technique may be used to generate therelationship between the calculated x and y chromaticity values (Δ's),or a typical curvature may be assumed from data sheets provided by anLED manufacturer. In other embodiments, however, a higher-orderinterpolation technique may be used to increase accuracy whencompensating certain colors of LEDs (e.g., green LEDs), which exhibitmore significant non-linear changes in chromaticity with drive current.

The x and y chromaticity values expected for each emission LED may beexpressed as a color point in the form of (x_(i), y_(i)). In anillumination device comprising four emission LEDs, step 42 of thecompensation method may result in the generation of four expected colorpoints: (x₁, y₁), (x₂, y₂), (x₃, y₃), and (x₄, y₄). Once the expectedcolor points are determined in step 42, the compensation method maycalculate the relative lumens needed from each of the emission LEDs toachieve the target luminance (Ym) and the target chromaticity setting(xm, ym) in step 44. For example, when light from four emission LEDs iscombined, the target luminance (Ym) of the combined light may beexpressed as:Ym=Y ₁ +Y ₂ +Y ₃ +Y ₄  (EQ. 2)where Y₁, Y₂, Y₃, and Y₄ represent the relative lumens of the fouremission LEDs. The relative lumen values (Y₁, Y₂, Y₃ and Y₄) may becalculated using well-known color mixing equations, the target luminance(Ym) and target chromaticity (xm, ym) values set within the illuminationdevice, and the expected color points (x₁, y₁), (x₂, y₂), (x₃, y₃), (x₄,y₄) determined in step 42 of the compensation method. As these equationsare well-known and readily understood by a skilled artisan, furtherdescription of such equations will be omitted herein.

Once the relative lumens (e.g., Y₁, Y₂, Y₃, and Y₄) are calculated foreach emission LED (in step 44), the drive currents that should besupplied to each emission LED to achieve the target luminance (Ym) aredetermined in step 46 similar to the manner described above. However,instead of determining a drive current (Ix) needed to achieve a “desiredluminous flux” (Lx) from each emission LED, the current embodimentdetermines the drive current (Ix) needed to achieve the relative lumenvalue (Y₁, Y₂, Y₃ or Y₄), which was calculated for each emission LED instep 44. Once the individual drive currents (Ix) are determined in step46, the emission LEDs are driven with the determined drive currents toproduce illumination having a desired luminous flux and a desiredchromaticity in step 48.

The compensation method shown in FIG. 8 may be used to adjust the drivecurrents supplied to the emission LEDs whenever a significant change inthe temperature is detected (in step 32) and/or the target luminance ortarget chromaticity setting is changed (in step 34). However, sincechanges in drive current inherently affect the LED junction temperature,the method steps shown in FIG. 8 may generally be repeated a number oftimes until the temperature stabilizes and the target luminance andchromaticity values remain unchanged.

One embodiment of a compensation method for controlling an LEDillumination device over changes in drive current and temperature hasnow been described with reference to FIGS. 8-12. Although thecompensation method described herein references target chromaticitysettings and chromaticity calibration values from the CIE 1931 XYZ colorspace, one skilled in the art would readily understand how thecompensation method could be modified to use target chromaticitysettings and chromaticity calibration values from other color spaces,such as the CIE 1931 RGB color space, the CIE 1976 LUV color space, andvarious other RGB color spaces (e.g., sRGB, Adobe RGB, etc.). For thisreason, the compensation method described herein and recited in theclaims is considered to encompass any color space that can be used todescribe the gamut of an LED illumination device comprisingsubstantially any combination of emission LEDs as described herein.

The compensation method shown in FIG. 8 provides many advantages overconventional compensation methods. As noted above, conventional methodstypically measure forward voltages by applying operative drive currentlevels to the emission LEDs. Unfortunately, forward voltages measured atoperative drive current levels vary significantly over the lifetime ofan LED. As an LED ages, the parasitic resistance within the junctionincreases, which in turn, causes the forward voltage measured atoperating current levels to increase over time, regardless oftemperature. For this reason, the present compensation method uses arelatively small drive current (e.g., about 0.1 mA to about 10 mA) toobtain forward voltage measurements from each LED individually, whileturning off all emission LEDs not currently under test. This improvesthe accuracy of the operating forward voltage values and enables eachemission LED to be individually compensated for temperature and process.

Conventional methods often rely on typical values or linearrelationships between luminous flux and drive current when performingtemperature compensation. In contrast, the compensation method describedherein interpolates between a plurality of stored luminous fluxcalibration values taken at different drive currents and differenttemperatures, and derives a non-linear relationship between luminousflux and drive current for each LED at the present operating temperature(Vfe_present). This enables the present compensation method toaccurately and individually characterize the luminous flux vs. drivecurrent relationship for each LED included within the illuminationdevice, and to provide accurate temperature compensation, regardless ofprocess. As a consequence, the compensation method described hereinprovides more accurate control of the luminous flux over temperaturechanges.

The compensation method shown in FIG. 8 further improves uponconventional compensation methods by providing a more accurate method ofcontrolling the luminance and chromaticity of the illumination devicewhen the dimming level or color point setting is changed by a user orbuilding controller. When a change in target luminance or targetchromaticity is detected by the illumination device, the drive currentssupplied to the emission LEDs must be adjusted to achieve the new targetvalue(s). The compensation method described herein increases theaccuracy with which new drive currents are determined by interpolatingbetween a plurality of stored chromaticity calibration values taken atdifferent temperatures, and deriving a relationship between chromaticityand drive current for each emission LED at the present operatingtemperature (Vfe_present). This enables the present compensation methodto accurately determine the expected chromaticity values that should beproduced for each emission LED at the present operating temperature andthe present drive current. As a consequence, the compensation methoddescribed herein provides more accurate control of the chromaticity asdrive current changes.

While the compensation method shown in FIG. 8 provides an accuratemethod for controlling the luminous flux and chromaticity of an LEDillumination device over changes in drive current and temperature, itdoes not account for LED aging effects. In order to mitigate sucheffects, FIG. 13 provides an improved method for maintaining a desiredluminous flux and a desired chromaticity over time, as the LEDs age. Asnoted above, the compensation methods shown in FIGS. 8 and 13 may beused alone, or together to provide accurate compensation for all LEDsused in the illumination device over changes in drive current,temperature and time.

In general, the compensation method shown in FIG. 13 may be performedrepeatedly throughout the lifetime of the illumination device to accountfor LED aging effects. The method shown in FIG. 13 may be performed atsubstantially any time, such as when the illumination device is firstturned “on,” or at periodic or random intervals throughout the lifetimeof the device. In some embodiments, the compensation method shown inFIG. 13 may be performed after a change in temperature, dimming level orcolor point setting is detected to fine tune the drive current valuesdetermined in the compensation method of FIG. 8.

As shown in FIG. 13, the age compensation method may generally begin bydriving the plurality of emission LEDs substantially continuously toproduce illumination, e.g., by applying operative drive currents (Idrv)to each of the plurality of emission LEDs (in step 50). As noted above,the term “substantially continuously” means that an operative drivecurrent is applied to the plurality of emission LEDs almostcontinuously, with the exception of periodic intervals during which theplurality of emission LEDs are momentarily turned off for shortdurations of time (in step 52). In the method shown in FIG. 13, theperiodic intervals may be used for obtaining various measurements from adedicated photodetector (e.g., a red photodetector) included within theillumination device or an emission LED (e.g., a red emission LED)configured to detect incident light. In step 54, for example, theperiodic intervals are used for measuring a photocurrent (Iph), which isinduced on the photodetector in response to illumination that isproduced by each emission LED, one LED at a time, and received by thephotodetector. In step 56, the periodic intervals are used for obtaininga forward voltage (Vfd), which develops across the photodetector uponapplying a relatively small (i.e., non-operative) drive current thereto.

FIG. 14 is an exemplary timing diagram illustrating steps 50, 52, 54 and56 of the compensation method shown in FIG. 13, according to oneembodiment of the invention. As shown in FIGS. 13 and 14, the pluralityof emission LEDS are driven substantially continuously with operativedrive current levels (denoted generically as I1 in FIG. 14) to produceillumination (in step 50 of FIG. 13). At periodic intervals, theplurality of emission LEDs are turned “off” for short durations of time(in step 52 of FIG. 13) by removing the drive currents, or at leastreducing the drive currents to non-operative levels (denoted genericallyas I0 in FIG. 14). Between the periodic intervals, the illuminationdevice produces continuous illumination with DC current supplied to theemission LEDs.

During some of the periodic intervals, one emission LED is driven withan operative drive current level (I1) to produce illumination, while theremaining LEDs remain “off,” and the photocurrent (e.g., Iph1) inducedin the photodetector by the illumination from the driven LED ismeasured. The photocurrents (e.g., Iph1, Iph2, Iph3, and Iph4) inducedin the photodetector by the illumination produced by each of theemission LEDs are measured, one LED at a time, as shown in FIG. 14 andstep 54 of FIG. 13. Although FIG. 14 provides an exemplary timingdiagram for an illumination device comprising four emission LEDs, suchas RGBY or RGBW, the timing diagram described herein can be easilymodified to accommodate a fewer or greater number of emission LEDs.

Sometime before or after the photocurrent (Iph) measurements areobtained, a forward voltage (Vfd) is measured across the photodetectorby applying a relatively small, non-operative drive current (e.g.,approximately 0.1-0.3 mA) to the photodetector (in step 56 of FIG. 13).This forward voltage measurement (also referred to herein asVfd_present) provides an indication of the current junction temperatureof the photodetector. Although the timing diagram of FIG. 14 shows onlyone forward voltage (Vfd) measurement obtained from a singlephotodetector, the timing diagram can be easily modified to accommodatea greater number of photodetectors.

In one exemplary embodiment, the presently described compensation methodmay be utilized within an illumination device comprising a plurality ofphotodetectors implemented with differently colored LEDs. In particular,each emitter module of the illumination device may include one or morered LEDs and one or more green LEDs as photodetectors. In such anembodiment, a forward voltage measurement (Vfd) may be obtained fromeach photodetector by applying a small drive current thereto (in step56). In some cases, the photocurrents associated with each emission LED(e.g., Iph1, Iph2, Iph3, Iph4) and the forward voltage(s) associatedwith each photodetector (Vfd) may be independently averaged over aperiod of time, filtered to eliminate erroneous data, and stored forexample in a register of the illumination device.

In addition to the photocurrents and detector forward voltage(s), theperiodic intervals shown in FIG. 14 may be used to obtain othermeasurements, such as the emitter forward voltages (Vfe1, Vfe2, etc.)described above with respect to FIGS. 8 and 9. The periodic intervalsmay also be used for other purposes not specifically illustrated herein.For example, some of the periodic intervals may be used by thephotodetector to detect light originating from outside of theillumination device, such as ambient light or light from otherillumination devices. In some cases, ambient light measurements may beused to turn the illumination device on when the ambient light leveldrops below a threshold (i.e., when it gets dark), and turn theillumination device off when the ambient light level exceeds anotherthreshold (i.e., when it gets light). In other cases, the ambient lightmeasurements may be used to adjust the lumen output of the illuminationdevice over changes in ambient light level, for example, to maintain aconsistent level of brightness in a room. If the periodic intervals areused to detect light from other illumination devices, the detected lightmay be used to avoid interference from the other illumination deviceswhen obtaining the photocurrent and detector forward voltagemeasurements in the compensation method of FIG. 13.

In other embodiments, the periodic intervals may be used to measuredifferent portions of a particular LED's spectrum using two or moredifferent colors of photodetectors. For example, the spectrum of aphosphor converted white LED may be divided into two portions, and eachportion may be measured separately during two different periodicintervals using two different photodetectors. Specifically, a firstperiodic interval may be used to detect the photocurrent, which isinduced on a first photodetector (e.g., a green photodetector) by afirst spectral portion (e.g., about 400 nm to about 500 nm) of thephosphor converted white LED. A second periodic interval may then beused to detect the photocurrent, which is induced on a secondphotodetector (e.g., a red photodetector) by a second spectral portion(e.g., about 500 nm to about 650 nm) of the phosphor converted whiteLED. As described in more detail below, dividing the spectrum of thephosphor converted white LED into two portions, and measuring eachportion separately with two different colors of photodetectors mayenable the compensation method of FIG. 13 to detect and account forchanges in chromaticity that occur as the phosphor converted LED ages.

Once the photocurrents and detector forward voltage(s) are measured, thecompensation method shown in FIG. 13 may determine the photocurrents(Iph_exp) that are expected for each emission LED (in step 58) using theforward voltage (Vfd_present) presently measured across thephotodetector, the drive current (Idrv) presently applied to the LED,the table of stored calibration values generated during the calibrationmethod of FIG. 6, and one or more interpolation techniques. FIG. 15depicts how one or more interpolation techniques may be used todetermine the expected photocurrent (Iph_exp) for a given LED at thepresent operating temperature (Vfd_present) and the present drivecurrent (Idrv) from the table of stored calibration values.

In FIG. 15, the solid dots (●'s) represent the photocurrent calibrationvalues, which were obtained during calibration at three different drivecurrents (e.g., 10%, 30% and 100% of the maximum drive current) and twodifferent temperatures (e.g., T0 and T1). As noted above, thephotocurrent calibration values (●'s) were stored within a table ofcalibration values for each emission LED included within theillumination device (see, e.g., FIG. 7). To determine the expectedphotocurrent value (Iph_exp) for a given LED, the compensation method ofFIG. 13 interpolates between the stored calibration values (●'s) tocalculate the photocurrent values (Δ's), which should be produced at thepresent operating temperature (Vfd_present) when using the same threedrive currents (e.g., 10%, 30% and 100% of the maximum drive current).

The change in photocurrent over temperature is non-linear for all LEDcolors, since both the emitted power and the responsivity of thephotodetector decrease linearly with temperature. For this reason, anon-linear interpolation technique may be used to calculate thephotocurrent values (Δ's) at the present operating temperature for allLED colors. In some embodiments, the non-linear interpolation techniquemay be a higher-order interpolation, a “poor man's” quadraticinterpolation, or an assumed curvature. In other embodiments, theresults of two different linear interpolations (e.g., a linearinterpolation between emitted power and temperature, and a linearinterpolation between detector responsivity and temperature) may bemultiplied together to calculate the photocurrent values (Δ's) at thepresent operating temperature.

If the drive current (Idrv) presently supplied to the emission LEDdiffers from one of the calibrated drive current levels, thecompensation method of FIG. 13 may apply another interpolation techniqueto the calculated photocurrent values (Δ's) to generate a relationshipthere between (denoted by a solid line in FIG. 15). In some cases, ahigher-order interpolation of the calculated photocurrent values (Δ's)may be used to generate a non-linear relationship between photocurrentand drive current. In other cases, a piece-wise linear interpolationcould be used to characterize the non-linear relationship between thecalculated photocurrent values, or a typical curvature could be assumedfrom data sheets provided by the LED manufacturer. From the generatedrelationship, the expected photocurrent value (Iph_exp) may bedetermined for the present drive current (Idrv).

Once expected photocurrents (Iph_exp) are determined for each emissionLED (in step 58), the compensation method shown in FIG. 13 calculates ascale factor for each emission LED (in step 60) by dividing thephotocurrent (Iph_exp) expected for each LED by the photocurrent (e.g.,Ipd1) measured for each LED. Next, the compensation method applies eachscale factor to a desired luminous flux value for each emission LED toobtain an adjusted luminous flux value for each emission LED (in step62). In some embodiments, the desired luminous flux value may be one ofthe relative lumen values (Y₁, Y₂, Y₃ or Y₄) calculated, e.g., in step44 of the compensation method shown in FIG. 8 to account for any changesin the target luminance (Ym) and/or target chromaticity (xm, ym)settings stored within the illumination device. Finally, the drivecurrents presently applied to the emission LEDs are adjusted (in step64) to achieve the adjusted luminous flux values if a difference existsbetween the expected and measured photocurrents for any of the emissionLEDs.

The compensation method described above and illustrated in FIG. 13provides an accurate method for adjusting the individual drive currentsapplied to the emission LEDs, so as to compensate for the degradation inlumen output that occurs over time as the LEDs age. By accuratelycontrolling the luminous flux produced by each emission LED, thecompensation method accurately controls the color of an LED illuminationdevice comprising a plurality of multi-colored, non-phosphor convertedemission LEDs. However, and as noted above, some embodiments of theinvention may include a phosphor converted emission LED (e.g., a whiteor yellow LED) within the emitter module. In such embodiments,additional steps may be taken to control the luminous flux andchromaticity of the phosphor converted LED over time.

Like non-phosphor converted LEDs, the luminous flux produced by aphosphor converted LED generally decreases over time. Unlikenon-phosphor converted LEDs, however, phosphor converted LEDs are alsosusceptible to changes in chromaticity over time, since the efficiencyof the phosphor degrades as the phosphor ages. When a phosphor convertedLED is included in the emitter module, the compensation method shown inFIG. 13 may be used to detect and account for chromaticity shifts thatoccur in the phosphor converted LED by separately measuring andcompensating for the chromaticity shifts caused by phosphor aging. Inorder to do so, two separate photodetectors may be included within theemitter module for measuring the photocurrents, which are induced by twodifferent portions of the phosphor converted LED spectrum. In someembodiments, the emitter module may include two different colors ofdedicated photodetectors (e.g., red and green photodetectors as shown inFIG. 17B) for measuring the photocurrents induced by the two differentportions of the phosphor converted LED spectrum (as shown, e.g., in FIG.21). In other embodiments, only one dedicated photodetector may beincluded within the emitter module (e.g., a red photodetector, as shownin FIG. 16B), and one of the emission LEDs (e.g., a green emission LED)may be used, at times, as an additional photodetector.

In order to detect and account for chromaticity shifts that occur overtime in a phosphor converted white LED, the different portions of thephosphor converted LED spectrum must first be calibrated. For example,the dedicated red photodetector and the dedicated green photodetector(or the green emission LED) may both be used in step 16 of thecalibration method of FIG. 6 to obtain photocurrent measurements fromthe phosphor converted white LED. The white LED-on-red detectorphotocurrent measurements are stored in the calibration table of FIG. 7as “Iph_d1”. The white LED-on-green detector photocurrent measurementsare stored in the calibration table of FIG. 7 as “Iph_d2”. The Iph_d2calibration values are italicized in FIG. 7 to show that they areoptional values, which may only be obtained when a phosphor convertedLED is included within the emitter module.

As described above and shown in FIG. 7, the Iph_d1 and Iph_d2calibration values may be obtained for a phosphor converted white LED ateach of the three different drive currents (e.g., 10%, 30% and 100% ofthe max drive current) and at each of the two different temperatures(e.g., T0 and T1). The white LED-on-green detector photocurrent (Iph_d2)measurements indicate the photocurrents that were induced in the greenphotodetector (or the green emission LED) by light emitted by the blueLED portion of the phosphor converted white LED (i.e., the first portionshown in FIG. 21). The white LED-on-red detector photocurrent (Iph_d1)measurements indicate the photocurrents that were induced in the redphotodetector by the light that passed through the phosphor portion ofthe phosphor converted white LED (i.e., the second portion shown in FIG.21).

Sometime before or after each Iph_d1 and Iph_d2 measurement is obtained,a forward voltage (Vfd1 or Vfd2) is measured across the dedicated redphotodetector and the dedicated green photodetector (or the greenemission LED) to provide an indication of the red detector junctiontemperature and the green detector junction temperature at each of thecalibrated drive current levels and ambient temperatures. The Vfd1 andVfd2 measurements may be stored within the calibration table, as shownin FIG. 7.

During the compensation method of FIG. 13, the white LED-on-red detectorphotocurrent (Iph_d1) and the white LED-on-green detector photocurrent(Iph_d2) can be measured as described in step 54, and the expectedphotocurrents (Iph_exp) for the white LED-on-red detector and the whiteLED-on-green detector can be determined as described in steps 56-58.Next, the expected photocurrents (Iph_exp) determined in step 58 can berespectively divided by the photocurrents measured in step 54 to producea white LED-on-red detector scale factor and a white LED-on-greendetector scale factor, as described in step 60. The white LED-on-greendetector scale factor provides an indication of how the relative lumensof the LED portion of the phosphor converted LED has changed over time.The white LED-on-red detector scale factor provides an indication of howthe relative lumens of the phosphor portion of the phosphor convertedLED has changed over time. In some embodiments, the white LED-on-greendetector scale factor and the white LED-on-red detector scale factor canbe used to control the luminous flux and chromaticity of the phosphorconverted white LED, as if it were two separate LEDs.

For example, the white LED-on-red detector scale factor can be appliedin step 62 to an overall desired luminous flux value for the phosphorconverted LED to account for lumen changes caused by LED and phosphoraging. In some cases, the overall desired luminous flux value for thephosphor converted LED may be determined by adding together the relativelumen values, which were separately calculated for the LED portion andthe phosphor portion of the phosphor converted LED in steps 40-44 of thecompensation method of FIG. 8. In order to calculate the relative lumenvalues for the LED portion and the phosphor portion separately, thecompensation method of FIG. 8 may use the luminous flux, x chromaticityand y chromaticity calibration values, which were separately stored inthe calibration table for each portion of the phosphor converted whiteLED spectrum, as described above with reference to FIG. 6.

To account for chromaticity shifts in the phosphor converted white LEDover time, the white LED-on-green detector scale factor can be comparedto the white LED-on-red detector scale factor. Based on such comparison,the actual chromaticity of the phosphor converted LED can be determinedusing well known color mixing equations, and the overall chromaticity ofthe illumination device can be maintained by adjusting the drivecurrents applied to all emission LEDs.

While the method described above provides an acceptable solution forcontrolling the luminous flux and chromaticity of a phosphor convertedLED over time, the chromaticity of the phosphor converted white LED maybe more accurately determined, in some embodiments, by accounting fordetector aging. For example, the photocurrents induced on the red andgreen photodetectors (or the green emission LED) by a given amount ofincident light generally decreases over time. Although such decrease istypically not large, it may cause the white LED-on-green detector scalefactor and the white LED-on-red detector scale factor to likewisedecrease over time, which in turn, decreases the accuracy with which thechromaticity of the phosphor converted white LED is adjusted. Toeliminate this source of measurement error, the scale factors generatedin step 60 for the phosphor converted LED may be adjusted, in someembodiments, to account for detector aging. This may be achieved, forexample, by using the blue emission LED as a reference for the phosphorconverted white LED.

In some embodiments, the dedicated red photodetector and the dedicatedgreen photodetector (or the green emission LED) may both be used in step54 of the compensation method of FIG. 13 to obtain Iph_d1 and Iph_d2photocurrent measurements from the blue emission LED. After measuringthe forward voltages measured across the red and green detectors in step56, steps 58 and 60 of the compensation method of FIG. 13 may berepeated to produce a blue LED-on-red detector scale factor and a blueLED-on-green detector scale factor for the blue emission/reference LED.In some embodiments, the blue LED-on-red detector scale factor may beapplied in step 62 to a desired luminous flux value for the blueemission/reference LED to adjust the luminous flux produced by the blueemission/reference LED. In some embodiments, the desired luminous fluxvalue for the blue emission/reference LED may be a relative lumen value,which is calculated as described in steps 40-44 of the compensationmethod of FIG. 8.

In order to account for detector aging in the phosphor converted LEDscale factors, the white LED-on-green detector scale factor may bedivided by the blue LED-on-green detector scale factor to produce awhite-over-blue-on-green scale factor ratio (WoBoG). Likewise, the whiteLED-on-red detector scale factor may be divided by the blue LED-on-reddetector scale factor to produce a white-over-blue-on-red scale factorratio (WoBoR). Next, the WoBoR scale factor ratio may be further dividedby the WoBoG scale factor ratio to produce a White Phosphor over WhiteBlue pump (WPoWB) ratio. The White Phosphor over White Blue pump (WPoWB)ratio provides an indication of how the spectrum of the phosphorconverted white LED changes over time and accounts for any degradationof the detector responsivity that may over time. In some embodiments,the WPoWB ratio may be used to adjust the overall chromaticity of thephosphor converted LED. Once the overall chromaticity of the phosphorconverted LED is set, the white-on-red detector scale factor may beapplied in step 62 to the overall desired luminous flux value for thephosphor converted white LED to account for age related lumen changes.

The compensation method shown in FIG. 13 and described above providesmany advantages over conventional compensation methods. As with theother methods described herein, the compensation method of FIG. 13improves the accuracy with which the detector forward voltage(s) aremeasured by applying a relatively small drive current (e.g., about 0.1mA to about 0.3 mA) to the photodetector(s). In addition, thecompensation method of FIG. 13 interpolates between a plurality ofstored photocurrent values taken at different drive currents anddifferent temperatures, and derives a non-linear relationship betweenphotocurrent and drive current for each emission LED at the presentoperating temperature (Vfpresent). This enables the present compensationmethod to accurately and individually characterize the photocurrent vs.drive current relationship for each individual LED, thereby providingaccurate age compensation for all emission LEDs included within theillumination device. Furthermore, the compensation method describedherein accounts for both emitter and detector aging, and providescompensation for chromaticity shifts that occur when phosphor convertedwhite emission LEDs age. As a consequence, the age compensation methoddescribed herein provides more accurate control of the luminous flux andchromaticity of the individual emission LEDs over time.

Exemplary Embodiments of Improved Illumination Devices

The improved methods described herein for calibrating and controlling anillumination device may be used within substantially any LEDillumination device having a plurality of emission LEDs and one or morephotodetectors. As described in more detail below, the improved methodsdescribed herein may be implemented within an LED illumination device inthe form of hardware, software or a combination of both.

Illumination devices, which benefit from the improved methods describedherein, may have substantially any form factor including, but notlimited to, parabolic lamps (e.g., PAR 20, 30 or 38), linear lamps,flood lights and mini-reflectors. In some cases, the illuminationdevices may be installed in a ceiling or wall of a building, and may beconnected to an AC mains or some other AC power source. However, askilled artisan would understand how the improved methods describedherein may be used within other types of illumination devices powered byother power sources (e.g., batteries or solar energy).

Exemplary embodiments of an improved illumination device will now bedescribed with reference to FIGS. 16-20, which show different types ofLED illumination devices, each having one or more emitter modules.Although examples are provided herein, the present invention is notlimited to any particular type of LED illumination device or emittermodule design. A skilled artisan would understand how the method stepsdescribed herein may be applied to other types of LED illuminationdevices having substantially different emitter module designs.

FIG. 16A is a photograph of a linear lamp 70 comprising a plurality ofemitter modules (not shown in FIG. 16A), which are spaced apart from oneanother and arranged generally in a line. Each emitter module includedwithin linear lamp 70 includes a plurality of emission LEDs and at leastone dedicated photodetector, all of which are mounted onto a commonsubstrate and encapsulated within a primary optics structure. Theprimary optics structure may be formed from a variety of differentmaterials and may have substantially any shape and/or dimensionsnecessary to shape the light emitted by the emission LEDs in a desirablemanner. Although the primary optics structure is described below as adome, one skilled in the art would understand how the primary opticsstructure may have substantially any other shape or configuration, whichencapsulates the emission LEDs and the at least one photodetector.

An exemplary emitter module 72 that may be included within the linearlamp 70 of FIG. 16A is shown in FIG. 16B. In the illustrated embodiment,emitter module 72 includes four differently colored emission LEDs 74,which are arranged in a square array and placed as close as possibletogether in the center of a primary optics structure (e.g., a dome) 76,so as to approximate a centrally located point source. In someembodiments, the emission LEDs 74 may each be configured for producingillumination at a different peak emission wavelength. For example, theemission LEDs 74 may include RGBW LEDs or RGBY LEDs. In addition to theemission LEDs 74, a dedicated photodetector 78 is included within thedome 76 and arranged somewhere around the periphery of the array. Thededicated photodetector 78 may be any device (such as a siliconphotodiode or an LED) that produces current indicative of incidentlight.

In at least one embodiment, photodetector 78 is an LED with a peakemission wavelength in the range of approximately 550 nm to 700 nm. Aphotodetector with such a peak emission wavelength will not producephotocurrent in response to infrared light, which reduces interferencefrom ambient light sources. In at least one preferred embodiment,photodetector 78 may comprise a small red, orange or yellow LED. In someembodiments, the dedicated photodetector 78 may be arranged to capture amaximum amount light, which is reflected from a surface of the dome 76from the emission LEDs having the shortest wavelengths (e.g., the blueand green emission LEDs).

In some embodiments, the emitter module 72 may include a phosphorconverted white (W) emission LED 74, and the dedicated photodetector 78may be used to measure the photocurrents (Iph_d1) that are induced bythe light that passes through the phosphor portion of the phosphorconverted LED. In order to measure the photocurrents (Iph_d2) induced bythe blue LED portion of the phosphor converted LED, the green emissionLED 74 may be configured, at times, as an additional photodetector.These photocurrent measurements may be stored within the calibrationtable of FIG. 7 and used within the compensation method of FIG. 13 tocompensate for chromaticity shifts caused by phosphor aging.

FIGS. 17A and 17B illustrate a substantially different type ofillumination device and emitter module design. Specifically, FIG. 17Adepicts an illumination device 80 having a parabolic form factor (e.g.,a PAR 38) and only one emitter module (not shown in FIG. 17A). As theseillumination devices have only one emitter module, the emitter modulesincluded in such devices typically include a plurality of differentlycolored chains of LEDs, where each chain includes two or more LEDs ofthe same color. FIG. 17B illustrates an exemplary emitter module 82 thatmay be included within the PAR lamp 80 shown in FIG. 17A.

In the illustrated embodiment, emitter module 82 includes an array ofemission LEDs 84 and a plurality of dedicated photodetectors 88, all ofwhich are mounted on a common substrate and encapsulated within aprimary optics structure (e.g., a dome) 86. In some embodiments, thearray of emission LEDs 84 may include a number of differently coloredchains of LEDS, wherein each chain is configured for producingillumination at a different peak emission wavelength. According to oneembodiment, the array of emission LEDs 84 may include a chain of fourred LEDs, a chain of four green LEDs, a chain of four blue LEDs, and achain of four white or yellow LEDs. Each chain of LEDs is coupled inseries and driven with the same drive current. In some embodiments, theindividual LEDs in each chain may be scattered about the array, andarranged so that no color appears twice in any row, column or diagonal,to improve color mixing within the emitter module 82.

In the exemplary embodiment of FIG. 17B, four dedicated photodetectors88 are included within the dome 86 and arranged around the periphery ofthe array. In some embodiments, the dedicated photodetectors 88 may beplaced close to, and in the middle of, each edge of the array and may beconnected in parallel to a receiver of the illumination device. Byconnecting the dedicated photodetectors 88 in parallel with thereceiver, the photocurrents induced on each photodetector may be summedto minimize the spatial variation between the similarly colored LEDs,which may be scattered about the array. The dedicated photodetectors 88may be any devices that produce current indicative of incident light(such as a silicon photodiode or an LED). In one embodiment, however,the dedicated photodetectors 88 are preferably LEDs with peak emissionwavelengths in the range of 500 nm to 700 nm. Photodetectors with suchpeak emission wavelengths will not produce photocurrent in response toinfrared light, which reduces interference from ambient light.

In at least one preferred embodiment, emitter module 82 includes aphosphor converted white (W) emission LED 84 and two different colors ofphotodetectors for measuring different portions of the phosphorconverted LED spectrum. In one example, the dedicated photodetectors 88may include one or more small red LEDs and one or more small green LEDs.In such an example, the dedicated red photodetector(s) 88 may be used tomeasure the photocurrents (Iph_d1) that are induced by the light thatpasses through the phosphor portion of the phosphor converted LED, whilethe dedicated green photodetector(s) 88 are used to measure thephotocurrents (Iph_d2) induced by the light emitted by blue LED portionof the phosphor converted LED. These photocurrent measurements (Iph_d1and Iph_d2) may be stored within the calibration table of FIG. 7 andused within the compensation method of FIG. 13 to compensate forchromaticity shifts caused by phosphor aging.

The illumination devices shown in FIGS. 16A and 17A and the emittermodules shown in FIGS. 16B and 17B are provided merely as examples ofillumination devices in which the improved calibration and compensationmethods may be used. Further description of these illumination devicesand emitter modules may be found in related U.S. patent application Ser.No. 14/097,339 and related U.S. Provisional Patent Application No.61/886,471, which are commonly assigned and incorporated herein byreference in their entirety. However, the inventive concepts describedherein are not limited to any particular type of LED illuminationdevice, any particular number of emitter modules that may be includedwithin an LED illumination device, or any particular number, color orarrangement of emission LEDs and photodetectors that may be includedwithin an emitter module. Instead, the present invention may onlyrequire an LED illumination device to include at least one emittermodule comprising a plurality of emission LEDs and at least onededicated photodetector. In some embodiments, a dedicated photodetectormay not be required, if one or more of the emission LEDs is configured,at times, to provide such functionality.

FIG. 18A is a side view of one embodiment of an improved emitter module90 comprising a plurality of emission LEDs 92 and one or more dedicatedphotodetectors 94, all of which are mounted on a common substrate 96 andencapsulated within a primary optics structure (e.g., a dome) 98. A heatsink 100 is coupled to a bottom surface of the substrate 96 for drawingheat away from the heat generating components of the emitter module 90.The heat sink 100 may comprise substantially any material withrelatively high thermal and electrical conductivity. In someembodiments, the heat sink 100 is formed from a material having athermal conductivity that ranges between about 200 W/(mK) and about 400W/(mK). In one embodiment, the heat sink is formed from a copper orcopper-alloy material, or an aluminum or aluminum alloy material. Theheat sink 100 may be a relatively thick layer ranging between about 1 mmand about 10 mm, and in one embodiment, may be about 3 mm thick.

Emitter module 90 may include substantially any number and color ofemission LEDs 92 and substantially any number and color of dedicatedphotodetectors 94. In one exemplary embodiment, the emission LEDs 92include one or more red LEDs, one or more blue LEDs, one or more greenLEDs and one or more white or yellow LEDs, as shown in FIGS. 16B and17B. The emission LEDs 92 may generally be arranged in an array near thecenter of the dome 98, and the dedicated photodetectors 94 may generallybe arranged about a periphery of the array. In one exemplary embodiment,the dedicated photodetectors 94 may include one or more red, orange,yellow and/or green LEDs. The LEDs used to implement the dedicatedphotodetectors 94 are generally smaller than the emission LEDs 92, andare generally arranged to capture a maximum amount of light that isemitted from the emission LEDs 92 and reflected from the dome 98. Insome embodiments, dedicated photodetectors 114 may be omitted if one ormore of the emission LEDs 112 are configured, at times, for detectingincident light.

The primary optics structure 98 may be formed from a variety ofdifferent materials and may have substantially any shape and/ordimensions necessary to shape the light emitted by the emission LEDs ina desirable manner. Although the primary optics structure 98 isdescribed herein as a dome, one skilled in the art would understand howthe primary optics structure may have substantially any other shape orconfiguration, which encapsulates the emission LEDs 92 and the at leastone photodetector 94. In some embodiments, the shape, size and materialof the dome 98 may be generally designed to improve optical efficiencyand color mixing within the emitter module 90.

In one embodiment, substrate 96 may comprise a laminate material such asa printed circuit board (PCB) FR4 material, or a metal clad PCBmaterial. However, substrate 96 may be formed from a ceramic material(or some other optically reflective material), in at least one preferredembodiment of the invention, so that the substrate may generallyfunction to improve output efficiency by reflecting light back out ofthe emitter module 90. In addition, substrate 96 may be configured toprovide a relatively high thermal impedance, or low thermalconductivity, in the lateral direction (i.e., the direction in the planeof the substrate). In one embodiment, substrate 96 may be formed from amaterial (e.g., aluminum nitride, AN) having a thermal conductivity lessthan or equal to about 150 W/(mK). In another embodiment, substrate 96may be formed from a material (e.g., an aluminum oxide Al₂O₃ material)having a thermal conductivity less than about 30 W/(mK), or material(e.g., a PCB laminate material) having a thermal conductivity less thanabout 1 W/(mK). The high thermal impedance, or low thermal conductivity,provided by substrate 96 in the lateral direction advantageouslyisolates the junction temperatures of the emission LEDs 92 and thephotodetectors 94, and avoids inaccurate Vfe and Vfd measurements.

In some embodiments, substrate 96 may be further configured to provide arelatively low thermal impedance, or high thermal conductivity, in thevertical direction (i.e., the direction perpendicular to the plane ofthe substrate). In particular, a relatively low thermal impedance path102 may be provided between each emission LED 92 and each photodetector94 to the heat sink 100. In addition to improving heat dissipation, thelow thermal impedance paths 102 enable the substrate 96 to maintain aconsistent temperature difference between the emitter and detectorjunction temperatures over operating conditions.

As noted above, alternative embodiments of the temperature compensationmethod shown in FIG. 8 may use the forward voltage (Vfd) measured acrossthe photodetector(s), instead of the forward voltages (Vfe) measuredacross the emission LEDs, to provide an indication of the currentoperating temperature. However, these alternative embodiments note thatthe detector forward voltage (Vfd) may only be used if the temperaturedifference between the emission LEDs and the photodetector(s) remainssubstantially the same over the operating temperature range.

The improved emitter module 90 shown in FIG. 18A maintains a relativelyfixed temperature difference between the emission LEDs 92 and thededicated photodetectors 94 by providing each of the emission LEDs 92and each of the photodetectors 94 with a low thermal impedance path 102to the heat sink 100. This may be achieved in a number of differentways. In the particular embodiment shown in FIG. 18A, a low thermalimpedance path 102 is provided by minimizing the thickness of thesubstrate 96 (which minimizes the vertical distance between the emissionLEDs 92, the photodetectors 94 and the heat sink 100), and by connectingeach of the emission LEDs 92 and each of the photodetectors 94 to theheat sink 100 with a plurality of thermally conductive lines 102. In oneexample, the thickness (T) of substrate 96 may range between about 300μm and about 500 μm.

In general, the plurality of thermally conductive lines 102 may comprisesubstantially any thermally conductive material. In some embodiments,the thermally conductive lines 102 are formed from a material having athermal conductivity that ranges between about 200 W/(mK) and about 400W/(mK). The material used for the thermally conductive lines 102 may bethe same material used for the heat sink 100, or may be different. Inone embodiment, the thermally conductive lines 102 are formed from analuminum, aluminum-alloy, copper or copper-alloy material. The pluralityof thermally conductive lines 102 may be formed by drilling verticalholes through the substrate (using any mechanical or optical means), andfilling or plating the holes (or vias) with a metal material using anyappropriate method. In some embodiments, each thermally conductive line102 may comprise a plurality (e.g., about 10-20) of densely packed vias,with each via being on the order of a couple of hundred microns wide.

If the emitter module 90 shown in FIG. 18A is utilized within anillumination device, the detector forward voltage (Vfd) may be usedwithin the temperature compensation method of FIG. 8 to provide a goodindication of the junction temperatures of the emission LEDs 92. Sincethe temperature difference between the emission LEDs 92 and thephotodetectors 94 is fixed, the detector forward voltage (Vfd) may bemeasured in step 38 of FIG. 8 (instead of the emitter forward voltages),and the emitter forward voltages (Vfe) may be calculated using thedetector forward voltage (Vfd) measurement and the emitter forwardvoltages (Vfe) stored in the calibration table of FIG. 7. This wouldadvantageously reduce the number of forward voltage measurements thatneed to be obtained during the temperature compensation method of FIG.8.

While the emitter module shown in FIG. 18A provides desirable thermalcharacteristics, it may not provide sufficient electrical isolationbetween the emission LEDs, photodetectors and heat sink. For both theemission LEDs and the photodetectors, the electrical contacts to eitherthe anode, cathode, or both (in a flip-chip LED design) are generallyprovided on the backside of the LEDs. These contacts cannot beelectrically coupled to the heat sink by directly connecting thecontacts to the heat sink with the metal lines shown in FIG. 18A. Inorder to provide electrical isolation between the LEDs and the heatsink, routing layers are provided in FIG. 18B for connecting a chain ofLEDs together and for connecting the LED anodes/cathodes to externalcontacts outside of the dome.

FIG. 18B is a side view of another embodiment of an improved emittermodule 110 comprising a plurality of emission LEDs 92 and one or morededicated photodetectors 94, all of which are mounted on a commonsubstrate 112 and encapsulated within a primary optics structure 98.Many of the components shown in FIG. 18B are similar to those shown inFIG. 18A. Like components are denoted with like reference numerals.

Emitter module 110 may include substantially any number, color andarrangement of emission LEDs 92 and substantially any number, color andarrangement of dedicated photodetectors 94. The emission LEDs 92 and thededicated photodetectors 94 may be similar to those described above, butare not limited to such. In some embodiments, one or more of thededicated photodetectors 94 may be omitted if one or more of theemission LEDs 92 are configured, at times, for detecting incident light.

The primary optics structure 98 may be formed from a variety ofdifferent materials and may have substantially any shape and/ordimensions necessary to shape the light emitted by the emission LEDs 92in a desirable manner. Although the primary optics structure 98 isdescribed herein as a dome, one skilled in the art would understand howthe primary optics structure may have substantially any other shape orconfiguration, which encapsulates the emission LEDs 92 and the at leastone photodetector 94. In some embodiments, the shape, size and materialof the dome 98 may be generally designed to improve optical efficiencyand color mixing within the emitter module 110.

Heat sink 100 is coupled to a bottom surface of the substrate 112 fordrawing heat away from the heat generating components of the emittermodule 110. The heat sink 100 may comprise substantially any materialwith relatively high thermal and electrical conductivity. In someembodiments, heat sink 100 is formed from a material having a thermalconductivity that ranges between about 200 W/(mK) and about 400 W/(mK).In one embodiment, the heat sink is formed from a copper or copper-alloymaterial, or an aluminum or aluminum alloy material. In someembodiments, the heat sink 100 may be a relatively thick layer rangingbetween about 1 mm and about 10 mm, and in one embodiment, may be about3 mm thick.

Emitter module 110 differs from emitter module 90, in at least oneaspect, by electrically isolating the electrical contacts of theemission LEDs 92 and the photodetectors 94 from the heat sink 100. Thisis achieved in the embodiment of FIG. 18B by utilizing a substrate 112having multiple layers. While an overall thickness (e.g., about 300 μmto about 500 μm) of the multiple layer substrate 112 may be similar tothe single layer substrate 96 shown in FIG. 18A, in some embodiments,the multiple layer substrate 112 shown in FIG. 18B is generally formedto include multiple routing and dielectric layers, which not onlyprovide electrical isolation between the electrical contacts of the LEDsand the heat sink, but also improve routing flexibility.

According to one embodiment, multiple layer substrate 112 may include afirst routing layer 114, a first dielectric layer 116, a second routinglayer 118 and a second dielectric layer 120. The first routing layer 114is coupled to the electrical contacts of the emission LEDs 92 and theone or more photodetectors 94, and may be formed on the first dielectriclayer 116. The first routing layer 114 may have a thickness that rangesbetween about 10 μm to about 20 μm, may be formed of a material (e.g., acopper or aluminum material, or an alloy thereof) having a thermalconductivity that ranges between 200 W/(mK) and about 400 W/(mK), andmay be formed by any well-known process on the upper surface of thefirst dielectric layer 116. For example, the first routing layer 114 maybe formed by printing or depositing metal lines on the upper surface ofthe first dielectric layer 116.

The first dielectric layer 116 is sandwiched between the first routinglayer 114 and the second routing layer 118 for electrically isolatingthe electrical contacts of the LEDs from the heat sink 100. In someembodiments, the first dielectric layer 116 may be a relatively thinlayer having a thickness between about 10 μm and about 100 μm, and maybe formed from a dielectric material having a relative permittivity thatranges between about 3 and 12. In one example, the first dielectriclayer 116 may be formed from an aluminum nitride material or an aluminumoxide material, but is not limited to such materials.

In addition to providing electrical isolation, first dielectric layer116 provides a relatively high thermal impedance in the lateraldirection by using a material with a relatively low thermalconductivity, which is less than about 150 W/(mK), and keeping thethickness of the layer small relative to the spacing between theemission LEDs and the photodetectors. In one exemplary embodiment, thefirst dielectric layer 116 may have a thickness of about 30 μm, and theemission LEDs 92 and photodetectors 94 may be spaced at least 200-300 μmapart on an upper surface of the substrate 112. Such an embodiment wouldprovide at least 10 times higher thermal conductivity in the verticaldirection than in the lateral direction.

The second routing layer 118 is coupled between the first dielectriclayer 116 and the second dielectric layer 120 and is generallyconfigured for routing signals between the first routing layer 114 andexternal electrical contacts (not shown) arranged outside of the primaryoptics structure. Like the first routing layer 114, the second routinglayer 118 may have a thickness that ranges between about 10 μm to about20 μm, may be formed of a material (e.g., a copper or aluminum material,or an alloy thereof) having a thermal conductivity that ranges between200 W/(mK) and about 400 W/(mK), and may be formed by any well-knownprocess on the upper surface of the second dielectric layer 120. Forexample, the second routing layer 118 may be formed by printing ordepositing metal lines on the upper surface of the second dielectriclayer 120. In order to route signals between the first routing layer 114and the second routing layer 118, vias 124 may be formed within thefirst dielectric layer 116. These vias may be formed in accordance withany known process.

In some embodiments, the second dielectric layer 120 may be coupledbetween the second routing layer 118 and the heat sink 100 and may begenerally configured for providing a relatively high thermal impedancein the lateral direction, and a relatively low thermal impedance in thevertical direction. In some embodiments, the second dielectric layer 120may be a relatively thick layer having a thickness between about 100 μmand about 1000 μm, which imparts rigidity to the emitter module 110. Insome embodiments, the second dielectric layer 120 may be formed from adielectric material having a relative permittivity that ranges betweenabout 3 and 12 and a thermal conductivity that is less than about 150W/(mK). In one example, the second dielectric layer 120 may be formedfrom an aluminum nitride material or an aluminum oxide material, but isnot limited to such materials.

The second dielectric layer 120 is similar to the substrate 96 shown inFIG. 18A, in that it provides a relatively high thermal impedance in thelateral direction by implementing the second dielectric layer 120 with amaterial having a thermal conductivity less than about 150 W/(mK), and arelatively low thermal impedance in the vertical direction by includinga plurality of thermally conductive lines 122, which extend verticallythrough the second dielectric layer 120 between the second routing layer118 and the heat sink 100. As noted above, the plurality of thermallyconductive lines 122 may be formed from a material having a thermalconductivity that ranges between about 200 W/(mK) and about 400 W/(mK),such as a copper or aluminum material, or an alloy thereof. Theplurality of thermally conductive lines 122 may be formed by drillingvertical holes through the second dielectric layer (using any mechanicalor optical means), and filling or plating the holes (or vias) with anappropriate metal material using any appropriate method. In someembodiments, each thermally conductive line 122 may comprise a plurality(e.g., about 10-20) of densely packed vias, with each via being a coupleof hundred microns wide.

In some embodiments, a third routing layer 126 may be coupled betweenthe second dielectric layer 120 and the heat sink 100. Unlike the firstand second routing layers, which comprise metal lines printed on thefirst and second dielectric layers, the third routing layer 126 mayextend substantially continuously across an upper surface of the heatsink 100 for improving the thermal contact between the plurality ofthermally conductive lines 122 and the heat sink 100 and improving heatspreading there across.

Like the substrate 96 shown in FIG. 18A, the multiple layer substrate112 shown in FIG. 18B provides good thermal isolation between theemission LEDs 92, and between the emission LEDs and the photodetectors,while providing rigidity and maintaining good overall thermalconductivity to the heat sink 100. This allows the detector forwardvoltage (Vfd) measurements to be used in lieu of emitter forward voltage(Vfe) measurements in the compensation method of FIG. 8, which reducesthe number of forward voltage measurements that need to be obtainedduring the compensation method.

The multiple layer substrate 112 shown in FIG. 18B also provides otheradvantages, which the single layer substrate 96 cannot provide. Forexample, substrate 112 includes multiple routing and dielectric layers,which improve routing flexibility for connecting chains of the emissionLEDs 92 together, and provides electrical isolation between theelectrical contacts of the emission LEDs and photodetectors and the heatsink 100.

The multiple layer substrate 112 can also be implemented somewhatdifferently in alternative embodiments of the invention. For example,instead of using dielectric or ceramic materials for layers 116 and 120,these layers could be using a laminate material such as a printedcircuit board (PCB) FR4 material, or a metal clad PCB material. However,since the thermal conductivity of laminate materials (e.g., less thanabout 1 W/(mK)) is much less than that of ceramic materials, using alaminate material in lieu of a ceramic material would reduce the thermalconductivity of layer 120. Regardless of the material used for layer120, thermal conductivity may be increased in some embodiments of theinvention by increasing the number of thermally conductive lines 122included under the LED array. While this approach would provide betteroverall thermal conductivity from the LED array to the heat sink, itwould provide worse thermal separation between the emission LEDs.

FIG. 19 is one example of a block diagram of an illumination device 110configured to accurately maintain a desired luminous flux and a desiredchromaticity over variations in drive current, temperature and time. Theillumination device illustrated in FIG. 19 provides one example of thehardware and/or software that may be used to implement the calibrationmethod shown in FIG. 6 and the compensation methods shown in FIGS. 8 and13.

In the illustrated embodiment, illumination device 110 comprises aplurality of emission LEDs 126 and one or more dedicated photodetectors128. The emission LEDs 126, in this example, comprise four chains of anynumber of LEDs. In typical embodiments, each chain may have 2 to 4 LEDsof the same color, which are coupled in series and configured to receivethe same drive current. In one example, the emission LEDs 126 mayinclude a chain of red LEDs, a chain of green LEDs, a chain of blueLEDs, and a chain of white or yellow LEDs. However, the presentinvention is not limited to any particular number of LED chains, anyparticular number of LEDs within the chains, or any particular color orcombination of LED colors.

Although the one or more dedicated photodetectors 128 are alsoillustrated in FIG. 19 as including a chain of LEDs, the presentinvention is not limited to any particular type, number, color,combination or arrangement of photodetectors. In one embodiment, the oneor more dedicated photodetectors 128 may include a small red, orange oryellow LED, as discussed above with respect to FIG. 16B. In anotherembodiment, the one or more dedicated photodetectors 128 may include oneor more small red LEDs and one or more small green LEDs, as discussedabove with respect to FIG. 17B. In some embodiments, one or more of thededicated photodetector(s) 128 shown in FIG. 19 may be omitted if one ormore of the emission LEDs 126 are configured, at times, to function as aphotodetector. The plurality of emission LEDs 126 and the (optional)dedicated photodetectors 128 may be included within an emitter module,as discussed above. In some embodiments, an illumination device mayinclude more than one emitter module, as discussed above.

In addition to including one or more emitter modules, illuminationdevice 110 includes various hardware and software components, which areconfigured for powering the illumination device and controlling thelight output from the emitter module(s). In one embodiment, theillumination device is connected to an AC mains 112, and includes anAC/DC converter 114 for converting AC mains power (e.g., 120V or 240V)to a DC voltage (V_(DC)). As shown in FIG. 19, this DC voltage (e.g.,15V) is supplied to the LED driver and receiver circuit 124 forproducing the operative drive currents applied to the emission LEDs 126for producing illumination. In addition to the AC/DC converter, a DC/DCconverter 116 is included for converting the DC voltage V_(DC) (e.g.,15V) to a lower voltage V_(L) (e.g., 3.3V), which is used to power thelow voltage circuitry included within the illumination device, such asPLL 118, wireless interface 120, and control circuit 122.

In the illustrated embodiment, PLL 118 locks to the AC mains frequency(e.g., 50 or 60 HZ) and produces a high speed clock (CLK) signal and asynchronization signal (SYNC). The CLK signal provides the timing forcontrol circuit 122 and LED driver and receiver circuit 124. In oneexample, the CLK signal frequency is in the tens of mHz range (e.g., 23MHz), and is precisely synchronized to the AC Mains frequency and phase.The SNYC signal is used by the control circuit 122 to create the timingused to obtain the various optical and electrical measurements describedabove. In one example, the SNYC signal frequency is equal to the ACMains frequency (e.g., 50 or 60 HZ) and also has a precise phasealignment with the AC Mains.

In some embodiments, a wireless interface 120 may be included and usedto calibrate the illumination device 110 during manufacturing. As notedabove, for example, an external calibration tool (not shown in FIG. 19)may communicate luminous flux and chromaticity calibration values to anillumination device under test via the wireless interface 120. Thecalibration values received via the wireless interface 120 may be storedin the table of calibration values within a storage medium 121 of thecontrol circuit 122, for example. In some embodiments, the controlcircuit 122 may use the calibration values to generate calibrationcoefficients, which are stored within the storage medium 121 in additionto, or in lieu of, the received calibration values.

Wireless interface 120 is not limited to receiving only calibrationdata, and may be used for communicating information and commands formany other purposes. For example, wireless interface 120 could be usedduring normal operation to communicate commands, which may be used tocontrol the illumination device 110, or to obtain information about theillumination device 110. For instance, commands may be communicated tothe illumination device 110 via the wireless interface 120 to turn theillumination device on/off, to control the dimming level and/or colorset point of the illumination device, to initiate the calibrationprocedure, or to store calibration results in memory. In other examples,wireless interface 120 may be used to obtain status information or faultcondition codes associated with illumination device 110.

In some embodiments, wireless interface 120 could operate according toZigBee, WiFi, Bluetooth, or any other proprietary or standard wirelessdata communication protocol. In other embodiments, wireless interface120 could communicate using radio frequency (RF), infrared (IR) light orvisible light. In alternative embodiments, a wired interface could beused, in place of the wireless interface 120 shown, to communicateinformation, data and/or commands over the AC mains or a dedicatedconductor or set of conductors.

Using the timing signals received from PLL 118, the control circuit 122calculates and produces values indicating the desired drive current tobe used for each LED chain 126. This information may be communicatedfrom the control circuit 122 to the LED driver and receiver circuit 124over a serial bus conforming to a standard, such as SPI or I²C, forexample. In addition, the control circuit 122 may provide a latchingsignal that instructs the LED driver and receiver circuit 124 tosimultaneously change the drive currents supplied to each of the LEDs126 to prevent brightness and color artifacts.

In general, the control circuit 122 may be configured for determiningthe respective drive currents needed to achieve a desired luminous fluxand/or a desired chromaticity for the illumination device in accordancewith one or more of the compensation methods shown in FIGS. 8 and 13 anddescribed above. In some embodiments, the control circuit 122 maydetermine the respective drive currents by executing programinstructions stored within the storage medium 121. In one embodiment,the storage medium may be a non-volatile memory, and may be configuredfor storing the program instructions along with a table of calibrationvalues, such as the table described above with respect to FIG. 7.Alternatively, the control circuit 122 may include combinatorial logicfor determining the desired drive currents, and the storage medium 121may only be used for storing the table of calibration values.

In general, the LED driver and receiver circuit 124 may include a number(N) of driver blocks 130 equal to the number of emission LED chains 126included within the illumination device. In the exemplary embodimentdiscussed herein, LED driver and receiver circuit 124 comprises fourdriver blocks 130, each configured to produce illumination from adifferent one of the emission LED chains 126. The LED driver andreceiver circuit 124 also comprises the circuitry needed to measureambient temperature (optional), the detector and/or emitter forwardvoltages, and the detector photocurrents, and to adjust the LED drivecurrents accordingly. Each driver block 130 receives data indicating adesired drive current from the control circuit 122, along with alatching signal indicating when the driver block 130 should change thedrive current.

FIG. 20 is an exemplary block diagram of an LED driver and receivercircuit 124, according to one embodiment of the invention. As shown inFIG. 20, the LED driver and receiver circuit 124 includes four driverblocks 130, each block including a buck converter 132, a current source134, and an LC filter 138 for generating the drive currents that aresupplied to a connected chain of emission LED 126 a to produceillumination and obtain forward voltage (Vfe) measurements. In someembodiments, buck converter 132 may produce a pulse width modulated(PWM) voltage output (Vdr) when the controller 154 drives the “Out_En”signal high. This voltage signal (Vdr) is filtered by the LC filter 138to produce a forward voltage on the anode of the connected LED chain 126a. The cathode of the LED chain is connected to the current source 134,which forces a fixed drive current equal to the value provided by the“Emitter Current” signal through the LED chain 126 a when the “Led_On”signal is high. The “Vc” signal from the current source 134 providesfeedback to the buck converter 132 to output the proper duty cycle andminimize the voltage drop across the current source 134.

As shown in FIG. 20, each driver block 130 includes a differenceamplifier 137 for measuring the forward voltage drop (Vfe) across thechain of emission LEDs 126 a. When measuring Vfe, the buck converter 132is turned off and the current source 134 is configured for drawing arelatively small drive current (e.g., about 1 mA) through the connectedchain of emission LEDs 126 a. The voltage drop (Vfe) produced across theLED chain 126 a by that current is measured by the difference amplifier137. The difference amplifier 137 produces a signal that is equal to theforward voltage (Vfe) drop across the emission LED chain 126 a duringforward voltage measurements.

As noted above, some embodiments of the invention may use one of theemission LEDs (e.g., a green emission LED), at times, as aphotodetector. In such embodiments, the driver blocks 130 may includeadditional circuitry for measuring the photocurrents (Iph_d2), which areinduced across an emission LED, when the emission LED is configured fordetecting incident light. For example, each driver block 130 may includea transimpedance amplifier 135, which generally functions to convert aninput current to an output voltage proportional to a feedbackresistance. As shown in FIG. 20, the positive terminal of transimpedanceamplifier 135 is connected to the Vdr output of the buck converter 132,while the negative terminal is connected to the cathode of the last LEDin the LED chain 126 a. Transimpedance amplifier 135 is enabled when the“LED_On” signal is low. When the “LED_On” signal is high, the output oftransimpedance amplifier 135 is tri-stated.

When measuring the photocurrents (Iph_d2) induced by an emission LED,the buck converters 132 connected to all other emission LEDs should beturned off to avoid visual artifacts produced by LED current transients.In addition, the buck converter 132 coupled to the emission LED undertest should also be turned off to prevent switching noise within thebuck converter from interfering with the photocurrent measurements.Although turned off, the Vdr output of the buck converter 132 coupled tothe emission LED under test is held to a particular value (e.g., about2-3.5 volts times the number of emission LEDs in the chain) by thecapacitor within LC filter 138. When this voltage (Vdr) is supplied tothe anode of emission LED under test and the positive terminal of thetransimpedance amplifier 135, the transimpedance amplifier produces anoutput voltage (relative to Vdr) that is supplied to the positiveterminal of difference amplifier 136. Difference amplifier 136 comparesthe output voltage of transimpedance amplifier 135 to Vdr and generatesa difference signal, which corresponds to the photocurrent (Iph_d2)induced across the LED chain 126 a.

In addition to including a plurality of driver blocks 130, the LEDdriver and receiver circuit 124 may include one or more receiver blocks140 for measuring the forward voltages (Vfd) and photocurrents (Iph_d1or Iph_d2) induced across the one or more dedicated photodetectors 128.Although only one receiver block 140 is shown in FIG. 20, the LED driverand receiver circuit 124 may generally include a number of receiverblocks 140 equal to the number of dedicated photodetectors includedwithin the emitter module.

In the illustrated embodiment, receiver block 140 comprises a voltagesource 142, which is coupled for supplying a DC voltage (Vdr) to theanode of the dedicated photodetector 128 coupled to the receiver block,while the cathode of the photodetector 128 is connected to currentsource 144. When photodetector 128 is configured for obtaining forwardvoltage (Vfd), the controller 154 supplies a “Detector_On” signal to thecurrent source 144, which forces a fixed drive current (Idrv) equal tothe value provided by the “Detector Current” signal throughphotodetector 128.

When obtaining detector forward voltage (Vfd) measurements, currentsource 144 is configured for drawing a relatively small amount of drivecurrent (Idrv) through photodetector 128. The voltage drop (Vfd)produced across photodetector 128 by that current is measured bydifference amplifier 147, which produces a signal equal to the forwardvoltage (Vfd) drop across photodetector 128. As noted above, the drivecurrent (Idrv) forced through photodetector 128 by the current source144 is generally a relatively small, non-operative drive current. In theembodiment in which four dedicated photodetectors 128 are coupled inparallel, the non-operative drive current may be roughly 1 mA. However,smaller/larger drive currents may be used in embodiments that includefewer/greater numbers of photodetectors, or embodiments that do notconnect the photodetectors in parallel.

Similar to driver block 130, receiver block 140 also includes circuitryfor measuring the photocurrents (Iph_d1 or Iph_d2) induced onphotodetector 128 by light emitted by the emission LEDs. As shown inFIG. 20, the positive terminal of transimpedance amplifier 145 iscoupled to the Vdr output of voltage source 142, while the negativeterminal is connected to the cathode of photodetector 128. Whenconnected in this manner, the transimpedance amplifier 145 produces anoutput voltage relative to Vdr (e.g., about 0-1V), which is supplied tothe positive terminal of difference amplifier 146. Difference amplifier146 compares the output voltage to Vdr and generates a differencesignal, which corresponds to the photocurrent (Iph_d1 or Iph_d2) inducedacross photodetector 128. Transimpedance amplifier 145 is enabled whenthe “Detector_On” signal is low. When the “Detector_On” signal is high,the output of transimpedance amplifier 145 is tri-stated.

As noted above, some embodiments of the invention may scatter theindividual LEDs within each chain of LEDs 126 about the array of LEDs,so that no two LEDs of the same color exist in any row, column ordiagonal (see, e.g., FIG. 17B). By connecting a plurality of dedicatedphotodetectors 128 in parallel with the receiver block 140, thephotocurrents (Iph_d1 or Iph_d2) induced on each photodetector 128 bythe LEDs of a given color may be summed to minimize the spatialvariation between the similarly colored LEDs, which are scattered aboutthe array.

As shown in FIG. 20, the LED driver and receiver circuit 124 may alsoinclude a multiplexor (Mux) 150, an analog to digital converter (ADC)152, a controller 154, and an optional temperature sensor 156. In someembodiments, multiplexor 150 may be coupled for receiving the emitterforward voltage (Vfe) and the (optional) photocurrent (Iph_d2)measurements from the driver blocks 130, and the detector forwardvoltage (Vfd) and detector photocurrent (Iph_d1 and/or Iph_d2)measurements from the receiver block 140. The ADC 152 digitizes theemitter forward voltage (Vfe) and the optional photocurrent (Iph_d2)measurements output from the driver blocks 130, and the detector forwardvoltage (Vfd) and detector photocurrent (Iph_d1 and/or Iph_d2)measurements output from the receiver block 140, and provides theresults to the controller 154. The controller 154 determines when totake forward voltage and photocurrent measurements and produces theOut_En, Emitter Current and Led_On signals, which are supplied to thedriver blocks 130, and the Detector Current and Detector_On signals,which are supplied to the receiver block 140 as shown in FIG. 20.

In some embodiments, the LED driver and receiver circuit 124 may includean optional temperature sensor 156 for taking ambient temperature (Ta)measurements. In such embodiments, multiplexor 150 may also be coupledfor multiplexing the ambient temperature (Ta) with the forward voltageand photocurrent measurements sent to the ADC 152. In some embodiments,the temperature sensor 156 may be a thermistor, and may be included onthe driver circuit chip for measuring the ambient temperaturesurrounding the LEDs, or a temperature from the heat sink of the emittermodule. In other embodiments, the temperature sensor 156 may be an LED,which is used as both a temperature sensor and an optical sensor tomeasure ambient light conditions or output characteristics of the LEDemission chains 126. If the optional temperature sensor 156 is included,the output of the temperature sensor may be used in some embodiments todetermine if a significant change in temperature is detected in step 32of FIG. 8.

One implementation of an improved illumination device 110 has now beendescribed in reference to FIGS. 19-20. Further description of such anillumination device may be found in commonly assigned U.S. applicationSer. Nos. 13/970,944, 13/970,964 and 13/970,990. A skilled artisan wouldunderstand how the illumination device could be alternativelyimplemented within the scope of the present invention.

It will be appreciated to those skilled in the art having the benefit ofthis disclosure that this invention is believed to provide an improvedillumination device and improved methods for calibrating andcompensating individual LEDs in the illumination device, so as tomaintain a desired luminous flux and a desired chromaticity overvariations in drive current, temperature and time. In addition, emittermodules having improved thermal and electrical characteristics are alsoprovided herein. Further modifications and alternative embodiments ofvarious aspects of the invention will be apparent to those skilled inthe art in view of this description. It is intended, therefore, that thefollowing claims be interpreted to embrace all such modifications andchanges and, accordingly, the specification and drawings are to beregarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A method for controlling an illumination devicecomprising a plurality of emission light emitting diodes (LEDs), whereinthe method comprises: applying respective drive currents to theplurality of emission LEDs to drive the plurality of emission LEDssubstantially continuously to produce illumination; periodically turningthe plurality of emission LEDs off for short durations of time;measuring a forward voltage presently developed across each emission LEDby applying a non-operative drive current to each emission LED, one LEDat a time, during the short durations of time the plurality of emissionLEDs are periodically turned off; and determining chromaticity valuesthat are expected for each emission LED using the forward voltagemeasured across each emission LED, the respective drive current suppliedto each emission LED, a table of stored calibration values correlatingforward voltage and drive current to chromaticity at a plurality ofdifferent temperatures, and one or more interpolation techniques.
 2. Themethod as recited in claim 1, wherein the non-operative drive currentranges between approximately 0.1 mA and approximately 10 mA.
 3. Themethod as recited in claim 1, further comprising monitoring a targetluminance setting and a target chromaticity setting of the illuminationdevice, and performing the steps of measuring and determining only whena change is detected in at least one of the target luminance setting andthe target chromaticity setting.
 4. The method as recited in claim 1,wherein the table of stored calibration values comprises chromaticitycalibration values corresponding to a CIE 1931 XYZ color space, a CIE1931 RGB color space, or a CIE 1976 LUV color space.
 5. The method asrecited in claim 1, wherein for each emission LED, the table of storedcalibration values comprises: a first plurality of x chromaticity valuesand a first plurality of y chromaticity values, which were previouslydetected from the emission LED upon applying a plurality of differentdrive currents to the emission LED during a calibration phase when theemission LED was subjected to a first temperature; a second plurality ofx chromaticity values and a second plurality of y chromaticity values,which were previously detected from the emission LED upon applying theplurality of different drive currents to the emission LED during thecalibration phase when the emission LED was subjected to a secondtemperature; a first plurality of forward voltage values, which werepreviously measured across the emission LED upon applying anon-operative drive current to the emission LED, before, during or aftereach of the different drive currents was applied to the emission LED,when the emission LED was subjected to the first temperature; and asecond plurality of forward voltage values, which were previouslymeasured across the emission LED upon applying a non-operative drivecurrent to the emission LED, before, during or after each of thedifferent drive currents was applied to the emission LED, when theemission LED was subjected to the second temperature.
 6. The method asrecited in claim 5, wherein for each emission LED, the step ofdetermining chromaticity values comprises: calculating a third pluralityof x chromaticity values corresponding to the forward voltage measuredacross the emission LED by interpolating between the first plurality ofx chromaticity values and the second plurality of x chromaticity values;generating a relationship between the third plurality of x chromaticityvalues, if the respective drive current supplied to the emission LEDdiffers from one of the plurality of different drive currents applied tothe emission LED during the calibration phase; and determining, from thegenerated relationship, an expected x chromaticity value for theemission LED corresponding to the respective drive current applied tothe emission LED.
 7. The method as recited in claim 6, wherein for eachemission LED, the step of determining chromaticity values furthercomprises: calculating a third plurality of y chromaticity valuescorresponding to the forward voltage measured across the emission LED byinterpolating between the first plurality of y chromaticity values andthe second plurality of y chromaticity values; generating a relationshipbetween the third plurality of y chromaticity values, if the respectivedrive current supplied to the emission LED differs from one of theplurality of different drive currents applied to the emission LED duringthe calibration phase; and determining, from the generated relationship,an expected y chromaticity value for the emission LED corresponding tothe respective drive current applied to the emission LED.
 8. The methodas recited in claim 7, wherein the steps of calculating a thirdplurality of x chromaticity values and a third plurality of ychromaticity values each comprise using a linear interpolation techniqueto interpolate between the first and second plurality of x and ychromaticity values.
 9. The method as recited in claim 7, wherein thestep of generating a relationship comprises applying a higher-orderinterpolation to the third plurality of x and y chromaticity values togenerate a non-linear relationship between x and y chromaticity anddrive current at the forward voltage measured across the emission LED.10. The method as recited in claim 7, wherein the step of generating arelationship comprises applying a piece-wise linear interpolation to thethird plurality of x and y chromaticity values to approximate anon-linear relationship between x and y chromaticity and drive currentat the forward voltage measured across the emission LED.
 11. The methodas recited in claim 7, wherein the step of generating a relationshipcomprises assuming a typical curvature from data sheets provided by anLED manufacturer.
 12. The method as recited in claim 7, furthercomprising calculating a relative luminous flux needed from eachemission LED to achieve a target luminance setting and a targetchromaticity setting of the illumination device, wherein the relativeluminous flux for each emission LED is calculated using the targetluminance setting, the target chromaticity setting, the expected xchromaticity value and the expected y chromaticity value determined fromthe generated relationships.
 13. The method as recited in claim 12,further comprising: determining a drive current needed to achieve therelative luminous flux calculated for each emission LED using theforward voltage measured across each emission LED, the table of storedcalibration values, which further correlates forward voltage and drivecurrent to luminous flux at the plurality of different temperatures, andone or more interpolation techniques; and driving each emission LED withthe determined drive current to produce illumination having thecalculated relative luminous flux.
 14. The method as recited in claim13, wherein for each emission LED, the table of stored calibrationvalues further comprises: a first plurality of luminous flux values,which were previously detected from the emission LED upon applying theplurality of different drive currents to the emission LED during thecalibration phase when the emission LED was subjected to the firsttemperature; and a second plurality of luminous flux values, which werepreviously detected from the emission LED upon applying the plurality ofdifferent drive currents to the emission LED during the calibrationphase when the emission LED was subjected to the second temperature. 15.The method as recited in claim 14, wherein for each emission LED, thestep of determining a drive current comprises: calculating a thirdplurality of luminous flux values corresponding to the forward voltagemeasured across the emission LED by interpolating between the firstplurality of luminous flux values and the second plurality of luminousflux values; generating a relationship between the third plurality ofluminous flux values, if the relative luminous flux calculated for theemission LED differs from one of the third plurality of luminous fluxvalues; and determining the drive current needed to achieve thecalculated relative luminous flux by selecting, from the generatedrelationship, a drive current corresponding to the calculated relativeluminous flux.
 16. The method as recited in claim 15, wherein the stepof calculating a third plurality of luminous flux values comprises usinga linear interpolation technique or a non-linear interpolation techniqueto interpolate between the first and second plurality of luminous fluxvalues, and wherein selection between the linear interpolation techniqueand the non-linear interpolation technique is independently made foreach emission LED based on a color of the emission LED.
 17. The methodas recited in claim 15, wherein the step of generating a relationshipcomprises applying a higher-order interpolation to the third pluralityof luminous flux values to generate a non-linear relationship betweenluminous flux and drive current.
 18. The method as recited in claim 15,wherein the step of generating a relationship comprises applying apiece-wise linear interpolation to the third plurality of luminous fluxvalues to approximate a non-linear relationship between luminous fluxand drive current.
 19. The method as recited in claim 15, wherein thestep of generating a relationship comprises assuming a typical curvaturefrom data sheets provided by an LED manufacturer.
 20. An illuminationdevice, comprising: a plurality of emission light emitting diodes(LEDs); a storage medium configured for storing a table of calibrationvalues correlating forward voltage and drive current to chromaticity andluminous flux at a plurality of temperatures for each of the pluralityof emission LEDs; an LED driver and receiver circuit configured forapplying respective drive currents to the plurality of emission LEDs todrive the plurality of emission LEDs substantially continuously toproduce illumination, periodically turning the plurality of emissionLEDs off for short durations of time, and applying a non-operative drivecurrent to each emission LED, one LED at a time, during the shortdurations of time to measure an operating forward voltage developedacross each emission LED; and a control circuit configured fordetermining whether a target luminance setting or a target chromaticitysetting for the illumination device has changed, and if so, fordetermining new respective drive currents needed to achieve the targetluminance setting and the target chromaticity setting using theoperating forward voltages measured across each emission LED, the tableof calibration values and one or more interpolation techniques.
 21. Theillumination device as recited in claim 20, wherein each emission LED isconfigured for producing illumination at a different peak wavelength.22. The illumination device as recited in claim 20, wherein thenon-operative drive current ranges between approximately 0.1 mA andapproximately 10 mA.
 23. The illumination device as recited in claim 20,wherein the table of calibration values stored within the storage mediumcomprises chromaticity calibration values corresponding to a CIE 1931XYZ color space, a CIE 1931 RGB color space, or a CIE 1976 LUV colorspace.
 24. The illumination device as recited in claim 20, wherein foreach emission LED, the table of calibration values comprises: a firstplurality of x chromaticity values and a first plurality of ychromaticity values, which were previously detected from the emissionLED upon applying a plurality of different drive currents to theemission LED during a calibration phase when the emission LED wassubjected to a first temperature; a second plurality of x chromaticityvalues and a second plurality of y chromaticity values, which werepreviously detected from the emission LED upon applying the plurality ofdifferent drive currents to the emission LED during the calibrationphase when the emission LED was subjected to a second temperature; afirst plurality of forward voltage values, which were previouslymeasured across the emission LED upon applying a non-operative drivecurrent to the emission LED, after each of the different drive currentswas applied to the emission LED, when the emission LED was subjected tothe first temperature; and a second plurality of forward voltage values,which were previously measured across the emission LED upon applying anon-operative drive current to the emission LED, after each of thedifferent drive currents was applied to the emission LED, when theemission LED was subjected to the second temperature; a first pluralityof luminous flux values, which were previously detected from theemission LED upon applying the plurality of different drive currents tothe emission LED during the calibration phase when the emission LED wassubjected to the first temperature; and a second plurality of luminousflux values, which were previously detected from the emission LED uponapplying the plurality of different drive currents to the emission LEDduring the calibration phase when the emission LED was subjected to thesecond temperature.
 25. The illumination device as recited in claim 24,wherein for each emission LED, the control circuit is further configuredfor: calculating a third plurality of x chromaticity valuescorresponding to the operating forward voltage measured across theemission LED by interpolating between the first plurality of xchromaticity values and the second plurality of x chromaticity values;generating a relationship between the third plurality of x chromaticityvalues, if the respective drive current applied to the emission LEDdiffers from one of the plurality of different drive currents applied tothe emission LED during the calibration phase; and determining, from thegenerated relationship, an expected x chromaticity value for theemission LED corresponding to the respective drive current applied tothe emission LED.
 26. The illumination device as recited in claim 25,wherein for each emission LED, the control circuit is further configuredfor: calculating a third plurality of y chromaticity valuescorresponding to the operating forward voltage measured across theemission LED by interpolating between the first plurality of ychromaticity values and the second plurality of y chromaticity values;generating a relationship between the third plurality of y chromaticityvalues, if the respective drive current applied to the emission LEDdiffers from one of the plurality of different drive currents applied tothe emission LED during the calibration phase; and determining, from thegenerated relationship, an expected y chromaticity value for theemission LED corresponding to the respective drive current applied tothe emission LED.
 27. The illumination device as recited in claim 26,wherein the control circuit is configured for calculating the thirdplurality of x and y chromaticity values for each emission LED by usinga linear interpolation technique to interpolate between the first andsecond plurality of x and y chromaticity values corresponding to theemission LED.
 28. The illumination device as recited in claim 26,wherein the control circuit is configured for applying a higher-orderinterpolation to the third plurality of x and y chromaticity values togenerate non-linear relationships between x and y chromaticity and drivecurrent for each emission LED.
 29. The illumination device as recited inclaim 26, wherein the control circuit is configured for applying apiece-wise linear interpolation to the third plurality of x and ychromaticity values to approximate non-linear relationships between xand y chromaticity and drive current for each emission LED.
 30. Theillumination device as recited in claim 26, wherein the control circuitis further configured for calculating a relative luminous flux neededfrom each emission LED to achieve the target luminance setting and thetarget chromaticity setting, and wherein the control circuit calculatesthe relative luminous flux for each emission LED using the targetluminance setting and the target chromaticity setting, and the expectedx chromaticity value and the expected y chromaticity value determinedfrom the generated relationships.
 31. The illumination device as recitedin claim 30, wherein for each emission LED, the control circuit isfurther configured for determining a drive current needed to achieve therelative luminous flux calculated for the emission LED using theoperating forward voltage measured across the emission LED, the table ofstored calibration values, and one or more interpolation techniques. 32.The illumination device as recited in claim 31, wherein the drivercircuit is further configured for driving each emission LED with thedetermined drive current to produce illumination having the calculatedrelative luminous flux.
 33. The illumination device as recited in claim31, wherein for each emission LED, the control circuit is furtherconfigured for: calculating a third plurality of luminous flux valuescorresponding to the operating forward voltage measured across theemission LED by interpolating between the first plurality of luminousflux values and the second plurality of luminous flux values; generatinga relationship between the third plurality of luminous flux values, ifthe relative luminous flux calculated for the emission LED differs fromone of the third plurality of luminous flux values; and determining thedrive current needed to achieve the calculated relative luminous flux byselecting, from the generated relationship, a drive currentcorresponding to the calculated relative luminous flux.
 34. Theillumination device as recited in claim 33, wherein the control circuitis configured for calculating the third plurality of luminous fluxvalues for each emission LED by using a linear interpolation techniqueor a non-linear interpolation technique to interpolate between the firstand second plurality of luminous flux values corresponding to theemission LED, and wherein selection between the linear interpolationtechnique and the non-linear interpolation technique is independentlymade for each emission LED based on a color of the emission LED.
 35. Theillumination device as recited in claim 33, wherein the control circuitis configured to generate the relationship by applying a higher-orderinterpolation to the third plurality of luminous flux values to generatea non-linear relationship between luminous flux and drive current foreach emission LED.
 36. The illumination device as recited in claim 33,wherein the control circuit is configured to generate the relationshipby applying a piece-wise linear interpolation to the third plurality ofluminous flux values to approximate a non-linear relationship betweenluminous flux and drive current for each emission LED.
 37. Theillumination device as recited in claim 33, wherein the control circuitis configured for generating the relationship by assuming a typicalcurvature from data sheets provided by a manufacturer of the emissionLED.
 38. The illumination device as recited in claim 20, furthercomprising a phase locked loop (PLL) coupled to an AC mains andconfigured for producing a timing signal in synchronization with afrequency of the AC mains, wherein the timing signal is supplied to theLED driver and receiver circuit for periodically turning the pluralityof emission LEDs off for the short durations of time.
 39. Theillumination device as recited in claim 20, further comprising atemperature sensor configured for detecting an ambient temperaturesurrounding the plurality of emission LEDs, and wherein if the targetluminance setting and the target chromaticity setting for theillumination device has not changed, the control circuit is furtherconfigured for determining the respective drive currents needed toachieve the target luminance setting and the target chromaticity settingonly when the ambient temperature changes by a specified amount.